The value of the UW engine on the MD is one of those perennial topics … I could say quite a bit, but you might look first at a few other places where we’ve discussed it before. Some really good answers among the posts in these threads:
MD Magic
MachineDrum with sampler redundant for Octatrack owner?
Just getting into these RAM machines!
Just got a Machine Drum - My initial thoughts/mini review
…among which somewhere you’ll no doubt encounter this classic video.
Short version, there are lots of good reasons to get a UW. Its sampling engine was the inspiration for the OT, but for most users, the more you work with it, the more broadly valuable you find it.