I have both. I love composing and creating tracks on the OP-1. I can’t do that (from scratch) on the Octatrack, the workflow just does not work for me. On the other hand, Octatrack is indispensable when performing, if you want to ditch the laptop. So what I do is I transfer stems into Octatrack, add some effects, some scenes, and I’m done!
For my next project I will combine some stems and some live looped guitar + effects on the Octatrack.
I actually sold the Octa, since I couldn’t compose with it, but re-acquired one recently when I realized I need one for live use. The second time has been much more pleasant.
Hope this is useful information to you. And yeah, get the MK I.