OT + MD
MD does percussions and drums in a way that the OT just can't do.
It has 16 tracks of percussive sounds that you can P-lock in a variety of ways that the OT is not designed for. You could even get a UW version and don't feel like you're doing the same things as on a OT.
Besides.. it's a classic!
ps. Love the OT as well! I got one as long as an A4.
Strangely enough, the OT right now does only wacky samples manipulation of "outer" source material (Kaoss, iPad, Radio and found sounds): no drums, no synths.