I’ve misplaced my Atmel “My Microprocessor can beat the Hell out of Your Microprocessor” sticker 
I don’t think it much matters. Whatever your comfy with. I started with AVR for the simple reason that I could build my own ‘no parts’ programmer and BascomAVR Basic compiler had a nice non-expiring demo (which is actually very nice and I highly recommend it). I am just now starting ARM and C.
I did find [this AVR vs PIC comparison to be rather unbiased.](http://onut.net/index2.php?option=com_content&do_pdf=1&id=25)