I thought I'd post my findings. I did some more googling and these results do NOT take into account electric bike use (faster braking speeds..etc.) but I thought I'd mention them anyways.
I found these links:
http://stores.ebay.com/International-Bike-Disc-Brake-Pads/About-Compounds.html
http://www.discobrakes.com/?s=0&t=8&q=compounds&
It looks like sintered pads are the best all-around. They last the longest and can take the most amount of abuse, they are quieter than semi-metallic, and brake better. They seem to deal with heat similar to semi-metallic pads so you can't really go wrong with it (semi-metallic is usually standard).
Kevlar pads have the best braking power but that comes at the cost of not lasting as long.
Ceramic pads come somewhere between semi-metallic and Sintered in terms of how long they last. They deal with heat better but aren't as quiet as sintered. Ceramics are about as loud as semi-metallic.
My own personal conclusion for when I change my brake pads: I'm going with Sintered. They perform better than semi-metallic and last much much longer. Same heat.
My ebike does 25MPH, but if I had a kit that went 35+ MPH i'd start taking Ceramic pads into account because they are better at dealing with heat and with faster speeds you're dealing with a lot more heat generated when breaking.
I don't think I'd look at kevlar unless your kit goes 20mph or less and you want the best performance and don't care about changing your pads more often.
I may be wrong on this one, but it looks like everything is better Semi-Metallic in every way. The only advantage of Semi metallic is that they're cheap, but only by a couple of bucks, not really worth to consider it at all.
Conclusion:
~25MPH kit, get sintered.
~35+MPH kit, get ceramic.
Don't get Semi-metallic.
Don't get kevlar unless you want the best braking power, don't care about changing pads often, and don't go too fast.
If I'm wrong in my conclusions feel free to chip in your own opinions!
If you have any questions on which type brake pads are best. We will give the professional answers to your questions.