I have been to Mid Ohio with 2 friends who have both the STi and the MS3. The MS3 guy was great with it. He would turn better times then classes above him in TT. The car was fast, handled ok, and was easy to drive. On the other hand it pushed like hell, sucked up gas, and would chew through race pads (2 weekends) like crazy.
The STi guy was only running in HPDE 2, so he was not pushing as hard, but he was pulling ok lap times. The car handled well, is hella quick and easier on the brakes.
Another friend had an Evo out there and he was driving it hard and not having problems.
The MS3 is a FF car that will not over steer and has lots of weight on the front tires and brakes. I would not track it for that reason.
I also don't like STi's and would get a RWD car for the track/fun car.
It is what you want. Sounds like a good deal on your trade and the STi price, but if you will not be able to track, dunno. Go drive the STi and see what you like better.
http://www.adamdaley.com/max/mr2/ben.jpg