The best one to get must be the "bluetooth" one, it's usually the most expemsive around $60 shipped. These ones are the most close ones to genuine PS3 controllers, it offers faster reaction, more precise motion, and only the top gamers would notice and appreciate the difference that worth $10 more.
For most the gamers (including myself), the 2.4G wireless (infrared technology) controllers are good enough, and you may save some bugs in the pocket :). The major difference between the 2.4 G wireless and bluetooth, is the later one has a little USB dongle to plug in the USB of the PS3 console. Without that the controller won't work. The major good thing for this kind of controllers, is it can be used as a wireless PC game pad as well, giving you a bit extra value for money, and convenience.
I assume that all the PS3 controllers should have dual shock3 and sixasix, otherwise you are missing the big part of fun. Some sellers sell the cheap controllers without those features, or even wired controllers, I do not even want to talk about these cheap ones, and I am a believer that these cheap ones should never be sold on ebay.