I think the following is as a rough guide:
Organic = good stoppers, faster wear
Sintered = not quite as much stopping power, but less heat fade, and more durable
Kevlar = compromise between the two
As with all these things, there are plenty of people on the internet saying ABC is amazing, and XYZ is poo, and just as many saying the opposite! Personally, I don't do any serious downhill, and try to ride smoothly rather than jumping on the brakes hard, so haven't really had any bad experiences with any type. I therefore tend to go for the cheapest ones from a brand I've heard of. My experiences aren't with BB7s, but the same principles still apply
PS: Don't know anything about ceramic pads. Semi-metal might be the same as sintered.