I fancied a 2.5 STi also but managed to find a 2006 06 plate STi PPP blob, registered early March 2006- cheap road tax!! In my opinion, and as posted above, it's down to personal taste- i just felt the 2.0 drove better and was overall the better car.
I've had mine 2 years and (touch wood) have had no problems at all with it. I have it serviced every 10k miles by my local dealer- the 3 year service has been the worst at £400, but that also included an MOT.
MPG varies, but i have seen 30-35 mpg on a long (and boring....) run. Usually it's down around 24 mpg combined.
I ran a focus ST2 at work (and also a ST Mountune) and the STi is a different animal to the focus. The Focus is still a good car, but there really isn't any comparison! I have also ran an EVO 6, 8, 9 and X at work and despite being awesome cars, i won't go into the details of running costs. If you buy one, have deep pockets.....
I don't regret buying the scoob for one minute and i use it daily for every scenario- i'm not one of these owners who hides the car away in a garage and only brings it out when the sun shines!! Mine see's rain, wind, snow and sun and has been on runs North of Inverness, across Ireland and down south on family trips- it never misses a beat.