If you are set on using Ableton, I would definitely recommend the Monomachine over the Octatrack.
I got an Octatrack a year ago and struggled to find a way to reintegrate it with Ableton. I even tried Cubase, Logic and Renoise but there were always issues (on the DAW side). Essentially you just need a host for soft synths and fx to run into the Octatrack - the rest is overlap. I ended up using Reaktor.
I wanted to switch from Ableton to a predominantly hardware setup because I felt that I was spending too much time obsessing over tiny details of the production rather than going with the flow and recording mixes as ‘takes’ like I used to do.
The Octatrack solves that problem by encouraging a performance-based approach. However, it didn’t stop me from getting stuck on listening to 4bar loops and I found it difficult to finish tracks without an overview of the entire timeline.
About 2 weeks ago, I got a Monomachine and everything just clicked. It helped that I had spent a year learning the Octatrack - most of the UI is the same, if not very similar. After a couple of days I had a workflow with the Octatrack sorted and was actually writing and finishing short pieces.
The Monomachine is absolutely incredible. It’s bursting with character and has an even more modular architecture than the Octatrack - there are so many ways to play it.
You may still struggle to find a way to integrate it into your Ableton workflow, but it won’t require you to completely change the way you record, as the Octatrack would.
If this is your first Elektron, you are going to want to switch off the computer altogether and just sit with the Monomachine and some headphones for the first few days / weeks. The secret is to give in to the Elektron way of doing things and not try to shoehorn it into your existing setup until you know what it is capable of.
Long-winded reply but, my advice is start out with the Monomachine and see where that takes you!
EDIT: Ah, you already have an MD. Missed that bit of your post. You’ll be up and running with the Mono in no time. If you’re unsure about the sound palette, get one 2nd hand and you can always resell it for next to no loss.