If power is your main concern then the Camaro is the way to go. The Camaro is faster in a straight line by a fair bit, not to say the MR2 is slow or anything. The MR2 is the better handling car, but the Camaro is isn't bad either. Both are upgradable, the Camaro can get 350rwhp with bolt-ons, the MR2 has the advantage of being a factory turbo car and will be easy to get more power from. The Camaro is rated at 17/26 mpg, and the MR2 is rated at 20/27 mpg. Looks are your opinion and therefore something you must decide for yourself. I'd go with the Camaro myself, it's more my style, it's loud and it's mean.
One other thing, if you want the power of the Camaro, but want to be a little different you could always opt for a Firebird.