Subaru is more reliable than Mitsubishi, but the Evo is a little more responsive to mods. The displacement difference of an STi vs Evo (2.5L vs 2.0L would probably mean that on the top end, you'll have more power out of the STi - don't quote me on this, though). Evo is a harsher ride, with an inferior (with exception of seats) interior - though, the interior of both cars leaves something to be desired.
All-in-all, they're about as close a comparison as they come, and the choice of Evo vs. STi is all about personal preference. To get an Evo with a 6-speed (which his how these cars should be IMHO), however, you have to buy the MR, which was always more expensive than the STi. You can get a Protune StageII for the STi for the cost of the Evo MR new, and have a leg up on the competition.
The Evo is not a bad car, but it wasn't the car for me. Seeing as though performance-wise, they're pretty similar, my choice was based on the aesthetics of the vehicle, which lead me to purchase the Subaru.