There's no right or wrong. The 2.5 has its issues and the fact a fair few of them have gone, there is obviously more of an issue than with the 2.0. However, having owned a 2006 hawk STI for 7 years and also owning a 2005 Blob STI (2.0), personally I preferred the 2.5. Mine was standard for a couple of years and then gradually increase and when I sold it it was running about 350bhp/380lbsft with absolutely no issues whatsoever. I was fairly sympathetic with it (warming up and cooling down etc), but maybe I was lucky. I would have another in a shot, but have to accept it could go pop, but then so could any engine. Just a good excuse to rebuild it!.
I would keep an open mind and look for the cleanest, low mile car with a complete history. That's what I'll be doing at the end of the year
Steve