Hey,
I have Onsong and use MM to control it, however, the context I use it in is generally for covers which I have created the total backing tracks including the drums.
The Beat Buddy works best (for me anyway) as time master. I am currently using with Ableton for open ended looping etc. MM works excellent for controlling both BB and Ableton into one cohesive environment (requires a bit of set setup). I keep BB on a stand to select beats and tempo. MM can change tempo also if needed during songs. There are a lot of ways to set it up how you want.
Have had MM for about a month. It’s the best foot controller I’ve had so far except for the number of custom modes available on the unit.
Onsong asks extra for a lot of upgrades. The midi upgrade is worth it. I have used it for a long time. It’s good at organizing lyrics and sets but there is a lot I don’t like about it.