Sti: 2.5L turbo running at 14psi stock. controllable center differntial
Evo: Forged internals, 2.0L 4g63t (i think that is the motor code), 19 psi stock.
the mitsu motor is a proven motor that is capable of ridiculous boost and really great power (same motor that was on the Eclipse GST and GSX and also used in the Talon TSi.). the scoobie motor i don't know too much about.
When you look at the two I would chose the Scoobie for the following reasons. The boost is lower stock. The motor has greater displacement. Although the mitsu motor is a proven motor it is already pretty high strung, and even with the forged internals the expandablility seems to be less. If you were to give the STi forged internals and run it at 19psi the STi would be smacking the piss out of the EVO. I guess it depends on what you plan in the future.
I would buy the scoobie because i liked the WRX and i feel the scoobie gives you some more room to work with. Either car is a great choice within that market. If you have time i would hold out a year or so cause i have heard the STi will be moving to a 3.0l motor in the next MY. It is just a rumor, but i would be willing to put stock in it. the 2.5L is having a hard time competing with the EVO so i could see subaru going nuts to win the war. If you buy either one i really, REALLY want to drive it.
Especially if it is the STi.. but i would settle for a ride.
BC