Knowledge and Prophecy are definitely a favorite for OTG. Scott also makes the Storm OTG which is a cheaper alternative. Also, the fan on the Storm is located inside of the goggle and doesn't have the battery pack on the goggle strap like the Smith Turbo Fan series does (Better if you wear goggles under your helmet). Hope this helps a bit.
if you keep your glasses clean they are fine. putting anything under the goggle/over my nose fogs up my glasses sometimes though. you can buy the little fans separately. They aren't too noisy, and i don't think the batteries would be too bad. especially if you wear a helmet, but i have never actually used one, just played with them at my shop.