As far as features and built in codes go, they're almost identical, but from a physical standpoint, the 15-2116 is much more popular than the 8910.
The 2116 looks alot like the enormously popular 15-1994 remote, but it lacks the four smart keys that most 1994 users use for macros, etc, which is a drag.
The 8910 has more buttons than the 2116, including some learning buttons, but it has a very nasty little SELECT button in the middle of a donut shaped ring, that you use for navigation, all very poor design IMHO. In fact, I wouldn't be at all surprised if they change that sometime soon.
Rob
http://www.hifi-remote.com