This past summer I shot with my normal collection of rangefinder gear. Then I had the great idea of using my CL and just using the 18mm and finder on it.
A dedicated camera to just one lens. The other cameras I would use in a normal fashion.
After a while I started to ask myself why am I carrying an entire camera for just the one lens. (Yes I know the CL takes other M-mount lenses but it doesn't have all the framelines and you need a viewfinder anyway. It's a somewhat limited M choice.)

So to answer your question. I think the 21mm with the dedicated camera is a nice option but would use the m-mount lens on my existing Leica body instead. It's less to carry, cost less, and gives you more flexibility.
Why carry a camera and lens when you could just add the lens to your line-up.

The ZM 21mm is the way to go.

-Rob