What do you want to know?
I've owned 2 Evos...loved the first, hated the second. That's pretty much how it is these days with Evos, though...you either get good ones that people have taken care of, or you get piles of shit with more issues than Brittany Spears. You REALLY have to do your research buying a used Evo. Ask if the person is on a forum, and go look at their posts...see if they have any "what is going on with this damn thing?" or "I raced a sweet bike, but now I have a CEL...what do?" threads. My friends are Evo people, and they are all very fast (one is in the 9s..another is in the mid 10s. One is built for AutoX/Road Racing, and the last is a daily driven Evo X...mostly stock). Two of them are actually doing the One Lap of America this year. They LOVE their cars, but have spent a small fortune getting them to be as fast as they are. It would be much EASIER to buy a new Mustang and make it fast than an EvoX and do the same.
I don't have any real experience with STi's, other than what I've been told. They like to understeer more than Evos, are a little slower, but are a little more comfortable. More difficult to get the same power out of an STi as an Evo. The 2009/2010 STi (the hatch, not the new Sedan) had engine issues. I'm not sure if they have fixed that with this newest one.
If I had infinite money, I would buy an Evo IX and built it to be bulletproof with 600awhp. However, I would have already bought a 2011 Mustang GT and built that first.
In either case, both cars will be fun to drive, pretty reliable when left nearly stock, functional (4 doors, plenty of cargo space), and mobile in all weather conditions. I STILL wish I would have kept my first Evo, but hey...hindsight is 20/20.
Evo #1 (Loved this thing)
Evo #2 (Hated this thing)