It depends on your budget. If you have several hundred dollars and want a stupid fast quad core, pick Intel. Was looking at their Core i7s a year ago, and those thing are quite amazing. http://bit.ly/6aIl4l If you're looking for a relatively fast, cheap processor, AMD. Budget forced me to pick up a 2.7 GHz dual core AMD last spring for $80 (mobo included). Run great so far, and its designed to overclock well. Haven't done it yet, but with factory fan (and the multiple fans on my case) I estimate I'll be able to run it at 3.2 GHz with little problem.
Go with what your budget lets you get.