OT.
The OT can be a drum machine, more easily than an MD can be a sampler.
The MD sampling feels more restricted. But I would have to list and compare every step of operation to explain, and I just woke up.
If the sampling functions are influencing your decision, I would get the OT first.
Both machines are different enough to be a powerful combination; neither can truly eliminate the other.
I got the OT first, and after a long time got the MD. I would not want to be without one or the other.
I do not think the MD will go truly obsolete for a long time - it is a unique machine. It could eventually go away simply because it has been with us for many years, but I don’t think that the AR will be the reason.
I think the OT is more versatile in almost every way. I’ve never played with single cycle waveforms - I have too much creative fun with samples, parameter locks and scenes.
If I could only have one machine, it would be the OT without question. But I love the MD and would not want to give it up.
Consider all of the above to be my opinions only.