STi or EVO?
Well, in 2006 the STi and EVO were pretty much matched stat for stat. If one was better than the other it was only by a margin, and the other would then excel at something else by a margin.
Those cars are so close to being the same thing, it's rather sickening. Aftermarket wise, the sky is the limit. Although admittedly the EVO has been seen to be made into monsters for what seems to be less monetary investment. But the over all picture is boiled down to this fact = buy which ever you think looks better.
MkIV Turbo Supra? Good luck finding one in good enough condition to consider buying for cheap. Not the best street car for 'racing'. The Supra is a GT car. It loves to cruise. That's what it's designed for. It just happens to be able to make power with little done to them. They're hard to make get traction under extreme launches and maintain what handling they do have.
Stick to the AWD cars if you're looking for the better buy. Unless you like racing on the freeway. Roll Patrol!