Oddly enough I was just talking to a professor and a TA about this topic earlier today and was coming here to see if anyone had commented on it. Sure enough, second post down. I'm going to go with AVR - I've used only PICs before, but the AVRs have so much pull due to the existance of a free compiler and cheap development platforms. Also, I can program them natively in OS X, rather than boot camping into Windows XP. I feel like, in some ways, PIC's are more on the windows side of the open/closed source dichotomy while AVR's are on the linux side. At any rate, from a hobbyist's standpoint, it certainly seems like AVR is the way to go. Though I will probably get a programmer for both!
-Colin