I favor the STi over the Evo any day. What the Impreza does with that 3 litre, 4 cylinder boxer engine is just awesome. Not only that, but on tarmac/asphalt tracks, I find the STi to be more sensitive. The Evo moves like a brick around the track, and thats from personal experience in the game. And putting the ridiculously huge hood scoop aside, I just like the way STi's look. I think Evo's are the most bland cars in the game in every aspect.
However, rally races are different. That lack of sensitivity the Evo has becomes more of a pro than a con. It glides easily through any track. But since GT4 allows us customization rights, thats a factor easily changed in the STi.