I have a DT and an OT and use my DT primarily for drum duty when i use it in conjunction with the OT.
As far as advantages i can think of a few:
I really like its form factor. Its half the size of the OT in width. I also am a huge fan of the two rows of 8 trigs as opposed to the long row of 16. Some may find its layout a bit cramped as opposed to the spaciousness of the OTs face, but as the DT was my first Elektron that was never a thing for me, rather, i find the OT a bit huge and things feel far apart ( i can cross the distance of the DT with one hand while some stuff on the OT takes two hands like using the FILL condition button press for instance)
DT is mono samples. I dont want to get into a whole thing about that as there are other threads, but mono samples are great for drums. The DT sounds punchy and crisp, not that the OT does NOT sound either punchy or crisp, but the DT has some mild high end boost that gives it a little je nais se quois…a little juju. I like that. Some folks dont, and to each their own.
I believe you mentioned being able to acquire the OT for even less than the DT so my next point is rather moot, but price. NOW is the time to get a used DT, with its beautiful gold screen. The prices they are being sold at are unreal, and getting such a powerful instrument at that price point is certainly a steal.
Learn the UI! If you havnt played one of the Digi series, then its quite different than the OT. Pattern based workflow as opposed to Part based on OT. THis can be a bit of a mind pretzel at first as you figure out the differences betwwen the two, but its worth it. Both have their advantages and disadvantages. As one who came from the DT first, it took me quite a while to understand why in the hell if i changed something on one pattern on the OT it changed it on all patterns for that track. drove me bonkers for a bit. Now i see ! and it makes a beautiful and logical sense. As does the DTs pattern based system. I love being able to teak a few parameters, close a filter on something, nudge some delay up or whatever and save these changes to a separate pattern and not have it affect that whole track through out the song im working on. LIke i said advantages to both and well worth checking out.
Freeing the OT from drum duty (insofar as drum duty requires more than a few tracks, at least before a resample) gives you a lot more freedom with what you want to do with the OT. I always have one track that resamples whatever is coming in on the DT, adds a bit of variance in terms of timbre, and then has some crossfader magic to keep the drums sounding alive and not static and stale.
You could absolutely do all this and waaaay more with a second OT.
Ive thought of going that route, but my mind recoils in horror at all the possibilities. Stare into the void and the void stares back into you.
Some annoying bullshit with the DT for drums:
You can not modulate the retrig paramaters. BOOO!
You can not plock the delay settings as they are a global send and not per track.
Ultimately, the OT is a way more powerful device and stands alone as a unique and mysterious instrument. But i will always speak up for the DT as a drum companion for it. DT is fast, fun, and very deep. Make drums on it and give it to the OT for the real spice.
Thats my two cents.