IMO Go w/ the WRX... you can make it a lot faster than a RSX will ever be for just a few thousand. They can actually take more boost than a stock blocked STI believe it or not... the weak point will be the tranny on the WRX once you get near 300 WHP.

Both honda and subaru make good cars, I own an STI and I'm on my fourth Honda in the form of an 03 SI hatchy, which is really a distant cousin of the RSX. I don't think you'll really regret either choice.