In some areas, the Evo is superior, but as a pure adrenaline pump, the STi has it beaten. I just love the motor on the STi. Its 2.5-liter, boxer four-cylinder, turbo-charged motor produces 305-hp and it sounds really good. In Sport Sharp mode it responds to your inputs very quickly (almost no turbo lag).
The Evo X claws back with better handling at the limit and a much better gearbox (if you pick the SST dual-clutch gearbox from the options menu).
So which is better? That is a very tough question to answer, but all I can say is, you will be very happy by picking either of these awesome machines.