How many different types are there?
Many... off the top of my head you have: rifle, carbine, H, H2, H3, 9mm, Endine, and the MGI RRB. Plus there are probably more I am not aware of.
What would be the reasons for changing, and how would you know that one is better than the other?
It depends on your use. Reasons for changing include to accelerate/delay the unlocking of the bolt (which can be both good and bad), to either prolong or shorten the recoil sensation, to increase/reduce reciprocating mass, etc.
Messing with buffers and springs is in general a good way to screw up the reliability of your rifle (assuming it worked to begin with). On the other hand, when you get that perfect combination, it is nice. Typically the major reasons you would mess with buffer weight are:
1. You bought an AR with a carbine gas system, 16" barrel and a standard carbine buffer and you want to delay the unlocking of the bolt so it is less violent, while at the same time adding reciprocating mass to help the rifle chug along when fouled.
2. You are running a suppressor and need a heavier buffer to delay unlocking due to the backpressure and more violent cycling created by the suppressor.
3. You shoot 3-gun and you are looking for that mystical combination of reliability and controllable recoil that gives you the best split times. For some guys this is as heavy a buffer as they can run reliably (they like a slow push for recoil). For others, it is as light a buffer as will reliably cycle the weapon (they like a short/fast cycle with less mass behind it).
Does anybody use any of the high tech buffers such as the endine?
Ran the MGI Rate Reducing Buffer. Waste of money for my uses. It helped a bit with my older carbine that had some wear and tear on it; but it caused more than its fair share of problems in my midlengths. Too heavy for that gas system.
Never had any issues with bolt bounce personally. Even the mid-tier manufacturers tend to use sliding weights which will counteract the tendency of the bolt to bounce. On the rare occasions I've read of it, someone is typically either trying to speed up the cyclic rate dramatically by lightening the buffer, or is messing with another caliber. However, not having had the problem, I can't say much about it.
I use a standard carbine buffer for midlengths (H-buffer if suppressed) and an H buffer in carbines (H2 if suppressed).
