There's a big difference between the two - in form/function as well as price. I have the Zen Vision with 30 GB HDD built in - and it accepts Compact Flash cards as well. Righ tnow it holds hundreds of MP3s, 5 movies converted to MPEG, and hundreds of digital images. I use it when I travel - lives in my shirt pocket. It also has a patch cable to connect it to a large TV.
The M is a lot smaller -can't speak for it.