I’ve owned an MD SPS1 non-UD and currently own an Octatrack. While the Octatrack can definitely cover percussion duties, it is vastly more interesting to tackle other jobs with it.
The MD I had was very well used and scored at a good price. That said, I regret selling it and someday will own another. Probably the UW, but that part won’t be a deal breaker. The MD’s synthesis engines is what I miss the most… I barely liked a sound on the default kits, a few tweaks in, the realization was clear, the MD is a BEAST.
Faced with the choice of an MD UW or an Octatrack, I’d have to go with the MD UW. But I’d probably add the Octatrack next.