My budget: Below $600
Dilemma: I don't know what a Midi/USB controller is, I have a Mac though, should I get one?
What I would use it for: I like to record myself and have it play over and over again. I would use it for performing also at weddings, etc. Needs to be carry-able.
What I want in it: Loop recording would be fun! Also, lots of voices, well as long as there are cool synth and string voices. Good response keys like hammer keys. AT LEAST 76 keys though. 88 would be perfect!

Was considering: Korg SP170, m-audio axio pro 61, Casio wk500, yamaha np60