i bought a 2010 limited rex, around 30k. i found a couple used sti's around, but it was really difficult to find any that were certified pre-owned, which is the only way i would go with that type of vehicle. the CPO program at subaru is really good, and it extends the warranty on the powertrain.
but, with the amount of modding that takes place with subarus, you may have a lot of trouble finding a clean and non-modded sti. i would be weary of buying one from a private seller unless you have a mechanic you trust that knows subarus that can inspect the vehicle to confirm that it was never modded.
if i were you, based on the fact that your rx8 is one of the best handling vehicles on the planet, i would probably lean towards the sti b/c it handles that much better than a stock wrx, but dont get into buying a junker that was all raced out and is not reliable.