STi vs. Evo: Even though I like the Lan Evos and the potential of the 4G/4B engines, I have to go with the WRX STi. Go WR Blue!
The Imprezas have a good amount of power STOCK compared to a Evo:
Lancer Evo IX MR CT9A = 290hp
Impreza WRX STi GDB = 300hp
Though with a Lancer Evo's 4G63 engine is a much stronger block than the EJ25 flat and can reach insane numbers with stock internals(some Evos have between 700-1000hp on stock parts and no rebuilt motor..) The Boxer4, espically when rebuilt with stronger parts than a 4G, reaches more hp numbers and torque than the 4G.
Stock or modded to the teeth, a STi has more torque than a Lan Evo.
Despite their differences, they are great cars, but I'm on the WR blue side

. I'm looking at a STi in real life as well and just can't get enough of that engine.
Corvette vs. Viper:
Mopar or no car!
The Corvettes are like the Skyline GTR/GTR of the US. Iconic and a legend that people respect around the world. The past recent C5, recent C6 and the soon-to-be C7, are powered with a torquey(?) yet powerful LS6/7/9 engine. In which can tear up any piece of road. A modified Z06/ZR1's LS engine can be deemed dangerous. But can't put that fear of a 8L V10 can.
A Dode Viper's 8-8.4L OHV V10 can put out more hp and torque than a LS any day. And a modified V10 with either a supercharged or twin turbo incuction can put fear in a Vette, similar to a beast 2JZ-GTE Supra on a RB26DETT Skyline. And a Viper twin turbo sounds nice, REAL nice.
But if I had the money, I'll get them both. But I will go with the Viper though, espically the new SRT.