So, as everyone seems to want the Alpina vs. M report, here goes.
Most are familiar with the whole story about how IMmie came to be in my possession and my massive "ooh, shiny" tendencies. For those that don't, the back story is
here.
First things first dear reader, we're comparing a 54 plate car with 45,000 miles to an 06 plate car with 6,000 miles on, so not a completely fair comparison. #69 was much loved and fully BMW maintained though, with no known faults when I sold her. IMmie is barely run in and like a brand new car.
From the outside, the pre-facelift vs facelift things are well known. IMmie has the aero boot spoiler whereas #69 had the standard Alpina front and rear spoilers. The Alpina spoilers seem to be a marmite thing, some love them some hate them. I prefer the M front spoiler and the Alpina rear. Contrary huh?
The Alpina rides on its standard 19" Dynamics which look a bitch to clean but are actually really well designed, as a hand with a wash mitt on will just fit nicely between the spokes. In my mind there's no doubt 19" wheels suit the Z4 better than 18" or smaller, but I have to say, the 19" wheels do suffer a little with tramlining (even with non run-flats) although keeping the tyre pressures spot on helps. The 18" wheels make the ride a little more forgiving on the M. I'm thinking about CSLs but I actually like the standard M wheels and the ride quality so maybe a little shadow chrome finish is in the offing. Mind you, if someone wants to sell a set of genuine CSLs, I'd pay good money and the current wheels will become winter wheels
Getting into the car, the biggest difference for me was when I closed the doors. The Alpina doors "clanged" and sounded cheap. The M doors "clunk" and that's what I'd have expected of both cars and, let's face it, any car that cost the thick end of £50K new (with options oh pedantic one). I don't know if this is an improvement to all facelift cars, or just the M but for build quality, the facelift car takes it for that alone.
Inside the cabin of both cars will be a familiar place to any Zed owner. Both cars have subtle differences to standard. The Alpina has a stitched leather dash with colour co-ordinated stitching to suit the car and the Alpina logo embossed onto the dash. #69 had blue and green and this is across the dash, the centre console and the steering wheel, some cars have red stitching. The M has the standard carbon leather trim. The Alpina dials and instrument panel are blue where the M has different coloured needles and the clocks illuminate in a different colour. The M has a fat, I mean chunky, steering wheel. Both cars have M seats. The Alpina has the logos in the seat backs, the M has the M logos on the head restraints. Both are nice places to be and I guess you pays your money, you takes your choice. In this case it's a dead heat.
Start up the cars and you start to notice the differences immediately. The Alpina starts up with a deep grumble, and as various valves move and the car warms up, it can sound for a minute or so like a bag of spanners. It alarmed me at first but I was assured it was normal and once you get used to it, the car has something of a personality as it wakes up. Start up the M and it sounds like a well oiled precision machine, it whirs. They have completely different exhaust notes and to be honest both make great noises. On the move the Alpina gets progressively louder building to a howl, not disimilar to a muscle car. The M is fairly quiet until the VANOS decides to turn on the taps and it makes a raucous rasping noise. They are different beasts indeed. With #69, if you pulled away before it had gone through its "cycle" it would kangaroo and misbehave, the M pulls cleanly, but you can tell it's cold. Not least of which the clue from the variable amber line on the rev counter.
On the move it's just chalk and cheese. The Alpina has a very progressive power delivery and as has been written elsewhere, you ride on a wave of torque that just doesn't go away. 0-60 is within 0.1 second of the M and you probably won't notice the difference. The M is raw performance, it's very very direct on its power delivery and it comes in with a bang as the revs hit the point where the valve timing shifts. Top speed, if you can find it, is higher with the Alpina by dint of it being unrestricted. Remove the M's restrictor (or hammer it in fifth gear) and I suspect the M will top out faster, but there won't be a lot in it. I'd love to be able to say that in the twisties they're well matched. I loved #69 but I have to say that the M is a better handling car when pushed. (now remember my comment about a 45,000 mile car compared to a 6,000 mile car) I've no idea if this is the limited slip diff, different suspension or a mixture of both, but I've driven both up to my personal limits on a road I am very familiar with and the M is more sure footed. There's no sport button on the Alpina, it's not needed. On the M I've only pushed it a few times. It seems directly wired to a smile

For the adventurous, the Alpina has the standard DTS/DSC options, the M has a mad button.
The steering is much discussed as the M has hydraulics and the Alpina uses the electronic power steering system. #69 suffered sticky steering and her steering column was replaced under warranty. The replacement was much lighter than the original but I actually liked it. Just after it was replaced I also had an alignment done and the geometry was set to the M settings as they didn't have the Alpina settings to hand

although it does make for a fairer comparison. The M steering definately adds to the general "planted" feel compared to the Alpina, but around town, in traffic or just parking, the Alpina is easier to live with.
The running costs of both are different too. Fuel economy should not be a consideration for anyone buying a 3 litre car should it, but the M drinks it like a Chelsea girl quaffs champagne. I was getting better than 33mpg from the Alpina, with no concession to driving economically. Driving the M the same way I get 26 mpg. I absolutely don't care! Neither car was bought as a daily drive and if I was worried about economy, I'd not have bought either of them. I'm lucky to be able to afford them as toys.
Anything else? Oh yes, the battery is in the boot on the M. Who cares, it doesn't have anything in the boot anyway.
So is one better than the other? I think they are different tools for different jobs. If you want a fast long distance hoon, the Alpina is probably a better car. It's more economical, and probably a little more forgiving. Over a long distance you'll probably get there faster too as the M owner will be stopping more often to fill up. However if you want a total smile machine that will give you an adrenaline buzz, and bite you in the arse if you get overly cocky with it, the M is your weapon of choice. The M is a better sports car in the truest meaning of the phrase.
I don't regret buying either and if I'd had the money I'd still own #69 as well as IMmie. I am glad I made the change though, IMmie is closer to what I was looking for when I first bought a zed, a total hooligan.