I wasn't very fond of UB for a long time but for the last 2 weeks, I started messing with it and have more appreciation for it. It can be good for beefing up your drums by synthesizing your own kicks, snares, etc.. and layering them over your samples. In addition, it's good for getting out your ideas real fast because of the sequence editor. You can always nudge and replace the drums, if need be. Also, you have a separate amp envelope for each sample, which the EXS lacks.
EXS is really powerful for melodic samples. Coming from a MPC 4000 and making keygroups in there for years, the EXS definitely kills it with that. However, I use battery for drums because I like the option of having a separate envelope for each sample and the MPC emulation.
Sorry if I got off topic or repeated the same response but that's just my .02