Peltier basics here:
http://www.overclockers.com/tips45/
Basically, a peltier consists of a cold plate that utilizes DC current to "pump" the heat from the cold side of the plate to the hot side. The more heat the peltier can pump, the larger amount of cooling you need on the hot side of the plate. Most people who run good peltier systems run water blocks to cool the hot side of the plate, as most of the time for any super cooling from the peltier, you're going to be dissipating alot of heat. Basically, like one diagram in that guide says, with a CPU generating 33 Watts of heat, and the peltier being able to move 78 watts, you're going to need a heatsink capable of removing 111 watts of heat. If you can't remove all the heat from the hot side, the hot side will just get hotter and hotter until the system shuts down or fries.
As for which is better, it all depends on how cold you NEED to get your system. Yes, you can get the CPU below ambient with a peltier, but keep in mind this means there will be condensation forming around the CPU socket, or anywhere else you have peltier cooling installed. And if you're going peltier, it's only worthwhile if you're going to water cool it as well, otherwise it won't be much better than conventional air cooling. But if you're talking something like this:
http://www.thermaltake.com/products/...bzero4g.htm#k7
Versus a water cooling system of good quality, I'd have to go with water cooling. There's too many other considerations you have to make with TEC, such as the extra juice required, condensation issues, and stuff like that.
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