Put it this way most of my images were taken with a 5 year old 10mp Canon 40D. (amazing little camera tho)
Doesnt matter what kit you have if you have the eye and know how to get the most out of your kit you can do anything!
Well im a Canon boy so I would go with the Canon. Nikon make good cameras but the glass is no where near as good as Canon. The EF lens mount is probably the best and most diverse system you can buy into.
The nikon being better in low light is so negligible that unless your a severe pixel peeper then you wouldn't notice, once its printed you wouldn't be able to tell the difference. More for the side by side 100% monitor comparisons which has so much detail that you can see minuscule differences. In the everyday there is very little between them, there might be 1/2 stop possibly. In the grand scheme of things this is no worry what so ever for average shooter. Could be for low light photographers that are shooting into the ridiculous ISO as normal practice like 6400-12800 and above. The Nikons are known at the moment for having better Dynamic range which means you can pull the shadows back, but if you expose the image correctly there isnt so much need for incredible DR. Its a pet hate of mine you find on the canon and nikon forums. But at the end of the day it is useful but for pros its a non issue which is why whenever you go to any event everyone is shooting canon. Lenses are better were known to be a tad cheaper, but now they are getting a little on the expensive side for new releases.
The 550D is a great little camera. The only thing it doesn't have compared to the 600 is the flipy screen otherwise its smaller, lighter and more compact. But I wouldn't buy a flipy screen simply because I would break it with the usage my cameras get. Great for getting different angles without getting all dirty lying on the floor. The 550D is a great camera, but the 60D offers a little more frames per second and a bigger body which I find more useful. I started with a 350D back in't day, so if you feel like you want something more you can alway upgrade later if you get into it more.
The advice I give to everyone starting out in photography is to buy a starter body then spend the money on lenses. Lenses are much more important than the body. Putting good glass on a camera is imperative. Also when you invest in lenses they outlive the cameras so once you have them you dont need to buy new versions.
Should be looking at lenses like the 15-85mm 17-55mm and 10-22mm specifically for the EF-S mount, L glass isnt a good idea for the wide end on a crop camera (which all the xxxD xxDs are) The sensor is smaller so if you use L glass you have to multiply the focal length by 1.6. A 24-105mm standard lens on a full frame camera would be 38.4-168mm 38mm is rubbish for wide angle on a crop body camera. Whereas it works out well on the longer end, a 70-200mm is a 112-320mm so you get more on zooms. Which is why I use crop cameras, I shoot a lot of motorsport so you get extra length without having to spend a fortune on the big whites.
A 70-200mm is 112-320 add a 2x extender you get 224-640mm although the quality does reduce slightly and the AF speed but its very useful.
Lenses on my wish list for your camera would be, in order:
17-55mm F2.8
10-22mm
70-200mm F4 IS
100mm 2.8 Macro IS
This is an expensive line up which you can upgrade to in time.
For now the twin lens kit is ok for starters but I would upgrade the standard zoom lens as quick as you can.
18-55mm 55-200mm
I would be looking to swap the 18-55mm to either the 15-85mm or if you can stretch to it the 17-55mm.
The 17-55mm and the 10-22mm are renowned for having L glass quality for crop cameras but without the weather sealing.
Hope this helps
And more than anything shoot as much as possible, can have the best gear in the world but if you dont learn the limits of your kit or shoot as much as possible then you cant get the best out of it.
I have just bought a 7D to fill in until I can get my hands on a 5D MKIII because my 40D finally gave up the ghost. I wouldn't buy a 7D for a long long time as I saw the ISO performance at low down was worse than the 6 year old 40D so I stuck with it. Now I pulled the trigger I can say yes it is a little noisier but it is an epic camera and people make out these tiny factors are huge problems when they are very minimal. Its a huge upgrade over the old cameras. Cant wait to get myself a 5D MKIII but the 4K price tag puts me off a tad for the shooting I do crop is better but want a full frame camera again after my 5D MKII went to a watery death lol.
Doesnt matter what kit you have if you have the eye and know how to get the most out of your kit you can do anything!

Well im a Canon boy so I would go with the Canon. Nikon make good cameras but the glass is no where near as good as Canon. The EF lens mount is probably the best and most diverse system you can buy into.
The nikon being better in low light is so negligible that unless your a severe pixel peeper then you wouldn't notice, once its printed you wouldn't be able to tell the difference. More for the side by side 100% monitor comparisons which has so much detail that you can see minuscule differences. In the everyday there is very little between them, there might be 1/2 stop possibly. In the grand scheme of things this is no worry what so ever for average shooter. Could be for low light photographers that are shooting into the ridiculous ISO as normal practice like 6400-12800 and above. The Nikons are known at the moment for having better Dynamic range which means you can pull the shadows back, but if you expose the image correctly there isnt so much need for incredible DR. Its a pet hate of mine you find on the canon and nikon forums. But at the end of the day it is useful but for pros its a non issue which is why whenever you go to any event everyone is shooting canon. Lenses are better were known to be a tad cheaper, but now they are getting a little on the expensive side for new releases.
The 550D is a great little camera. The only thing it doesn't have compared to the 600 is the flipy screen otherwise its smaller, lighter and more compact. But I wouldn't buy a flipy screen simply because I would break it with the usage my cameras get. Great for getting different angles without getting all dirty lying on the floor. The 550D is a great camera, but the 60D offers a little more frames per second and a bigger body which I find more useful. I started with a 350D back in't day, so if you feel like you want something more you can alway upgrade later if you get into it more.
The advice I give to everyone starting out in photography is to buy a starter body then spend the money on lenses. Lenses are much more important than the body. Putting good glass on a camera is imperative. Also when you invest in lenses they outlive the cameras so once you have them you dont need to buy new versions.
Should be looking at lenses like the 15-85mm 17-55mm and 10-22mm specifically for the EF-S mount, L glass isnt a good idea for the wide end on a crop camera (which all the xxxD xxDs are) The sensor is smaller so if you use L glass you have to multiply the focal length by 1.6. A 24-105mm standard lens on a full frame camera would be 38.4-168mm 38mm is rubbish for wide angle on a crop body camera. Whereas it works out well on the longer end, a 70-200mm is a 112-320mm so you get more on zooms. Which is why I use crop cameras, I shoot a lot of motorsport so you get extra length without having to spend a fortune on the big whites.
A 70-200mm is 112-320 add a 2x extender you get 224-640mm although the quality does reduce slightly and the AF speed but its very useful.
Lenses on my wish list for your camera would be, in order:
17-55mm F2.8
10-22mm
70-200mm F4 IS
100mm 2.8 Macro IS
This is an expensive line up which you can upgrade to in time.
For now the twin lens kit is ok for starters but I would upgrade the standard zoom lens as quick as you can.
18-55mm 55-200mm
I would be looking to swap the 18-55mm to either the 15-85mm or if you can stretch to it the 17-55mm.
The 17-55mm and the 10-22mm are renowned for having L glass quality for crop cameras but without the weather sealing.
Hope this helps

And more than anything shoot as much as possible, can have the best gear in the world but if you dont learn the limits of your kit or shoot as much as possible then you cant get the best out of it.

I have just bought a 7D to fill in until I can get my hands on a 5D MKIII because my 40D finally gave up the ghost. I wouldn't buy a 7D for a long long time as I saw the ISO performance at low down was worse than the 6 year old 40D so I stuck with it. Now I pulled the trigger I can say yes it is a little noisier but it is an epic camera and people make out these tiny factors are huge problems when they are very minimal. Its a huge upgrade over the old cameras. Cant wait to get myself a 5D MKIII but the 4K price tag puts me off a tad for the shooting I do crop is better but want a full frame camera again after my 5D MKII went to a watery death lol.