Wow, now that's a quandary. . . I can't directly compare but will add my thoughts seeing as how you asked! There's no concrete answer though!
On the road, the differences are not that important. The 3.0i is just as capable with a good driver of staying with an si and an ///M - it's only where driving become fit for the track that the difference is obvious. There is the different driving and ownership experience of course and having an ///M outside my front door does appeal! What doesn't appeal is the running costs which seem to double once you start with ///M ownership. Road Tax £500+, expensive tyres (the best tyres) and servicing which seems to literally be double that of the 3.0si and 3.0i. So it's not just the cost of the initial ///M, it's the on-going costs that I would have to take into account (as I do a lot of miles in my Zed) - including mpg.
So the 3.0si and 3.0i then become a more affordable Zed next to an ///M. Is there a difference in upgrading to the 3.0si from the 3.0i? I would say, if you were looking to buy a Zed then go for a 3.0si (if it's in your budget). If you already have a 3.0i then I wouldn't bother upgrading. The extra 34bhp only makes a difference for a very short time at the top of the rev range and unless your moving from a very early Zed (as you are) to something like an 08 or 09 then it doesn't make much sense. These are still, at best, 6 year old Zeds that need to be well maintained and you will have on-going costs and investments to make whether you have an 03 or an 09.
I would keep your 3.0i and your girlfriend happy (with a new kitchen) - in the long run it will be a far cheaper way of having a Zed which you can still have lots of fun and enjoyment with - knowing that the work is done to keep it fresh and capable, without losing thousands upgrading. Some on the forum state that the 3.0i engine is a more solid and reliable engine than that in the 3.0si and that Sterling grey was always the best Zed colour.
But then again, not many of us do the logical thing and often change our vehicles on a whim!
All the best with your decision - it's not going to be easy!
