Hi Andrew,
I think I'm qualified to speak on this having owned a heavily modified Sti and now an R32 GT-R. My only regret is that I binned the STi and would have liked to race that for a couple more years as it was only recently fully developed.
My view is that having tried both, it is far easier to make reliable horsepower with a GT-R, everything is off the shelf and of high quality.
WRX's often need a bit of trial and error as there are very few that are highly developed for club sprints. I can think of maybe 3 here in the Vic WRX Club. They are hit and miss to get right, it took me two years to get it where I wanted it.
The GT-R I have ran with a standard engine in club sporints for 2 years at 318 RWKW until it was rebuilt, mightly impressive considering it is 13 years old.
What I have found to be a good path to follow is buy a car that is not crash damaged and has sound suspension and gearbox, trim etc. If you want a cheap one, buy one with the crappiest engine you can find, as long as everything else is mickey mouse. Rebuild the engine and anciliaries for $15k - $20k. A $20k car plus a rebuild of $20k based on this theory will get you a brilliant road and track car and you are still at the price of a new WRX, with something that will have 350 RWKW and kill just about everthing on the track.
Come and have a chat to me at the next WRX track day if you want.
Regards
Andrew