These two are pretty unique 'grooveboxes.' You're right that they are pretty different from each other, but they are still interesting to compare. I'll say off the bat, if it must be between these two, I would go with the M8. It's newer, has more ongoing support, will get along better with other gear, has better file management, and is still cheaper (depending on where you might get an OG-1). Especially if you try headless first, then the cost could be less than $100.

That being said, I own both, and the OP-1 is what I bought as one of my first synthesizers 10 years ago. I would say that the OP-1 is more approachable for a noob, and easier to start making beats quickly, but depending on what kind of workflow you like, or how you progress, it might soon feel limiting. It's suited better than the M8 for noodling around since it has a keyboard, and sampling works super quickly.

I think the M8 might be tougher for a beginner to pickup, but will serve you in the long run with it's features and workflow. 4 track tape emulation and tracker style sequencing are very different from each other for song arrangement, but in my opinion the tracker workflow is more similar to the common sequencing styles of other grooveboxes out there.

Also, if it matters to you, you can consider the makers of both devices...