Again, being involved in both brands, I like the Cantabria more. It's not very common you have that many people in your spa, but I like the cooling-off step of the Cantabria and the layout more. (Personal preference.) Under the hood, expect them to be far more similar than different. The cabinet is the most obvious difference between both.
I can't speak of the M Series, as my competitor hasn't brought one in yet so I can't see it up close, looks nice, but I wouldn't put it as a higher class than the A9L they came out with last year. It looks like a STIL, and in our area, when I was a dealer, it just didn't sell that well, so something tells me I won't be seeing one here personally for a while