Definitely the STi; I WILL get one sometime in the future.
I got my hands on the show called Top Gear and they did a review on the cars. The Evo outperformed the STi just by a tad, but had flimsy interior; the STi has a better build quality. Also, at the end, the two hosts mentioned at the end of the day, after testing, they got in the STi to drive back to the studio instead of the Evo.
It's a matter of personal preference since they're so closely alike.