You'll get a lot of different answers here I'm sure. I've used the rubber kind that you have to slip the string through before you put them on and I didn't like them. I haven't used the leather ones. I settled on using the tip off of a rubber baby bottle nipple. The skin colored ones. I just cut the tip off of the nipple and slip it on the bow tip. Easy to get on and off and I never noticed any difference in the how the bow shot. They don't last as long as the other types but they are cheap and easy to replace and can easily be taken off to prevent moisture from building up under them.
Aaron