Working on memory, the head on the M300 is easily removed from the column. If necessary to avoid condensers shifting around, it should be possible to store the head in the same orientation as it sits on the column, so that's a plus. I can't remember how easy it is to remove the column from the base.
Conversely, my B22 (which I'd be happy to sell if it was feasible to ship) is taller. You can easily remove the head and column from the baseboard, but you need to keep head and column together. The condensers can be jostled around if not careful. So OK if you're storing the head and column upright in a closet, but not so good if storing horizontal.