02-05 WRXs are pretty slow stock. 2.0L block with a small turbo
06-07 WRXs are a bit quicker stock 2.5L block with a small turbo
08 WRX is pretty slow, same 2.5L block and small turbo, but i believe a bit heavier
09-13 WRXs are pretty quick, 2.5L block and a larger turbo (almost STi sized)
from 04-13 the STi has had about the same 2.5L block and turbo size. all rated to 300hp, all use a 6sp tranny with varying ratios and diff setups.
the 09-13 WRX are all underrrated from the factory at 265 hp. but independent back to back dyno testing by COBB, has shown that the 09-13 puts down on avg within 5hp (some cases more) of the 08-13 STi. gearing (less shifts) gives the WRX a slight lead on the STi for 0-60 times.
whatever you do, dont buy the 08 WRX. its slow and looks like shit (the most bland styling ever, no fender flares, no spoiler, no premium bumpers, small turbo)
IMO the 2011 WRX is the best looking one. only because it has the same wide fenders as the STi, and has the upgraded bumpers (for 2011+, even STi), and has the black headlights. the 2012-13 has chromed out headlights that i personally do not like.
my .02