In the next ISO respin, which will/should feature UP 6, MATE and Cinnamon will be separate. The problem with selecting one is that if the ISO comes with both, then you have to download a large ISO with both even if you only want one. Also, if an ISO comes with MATE, but it gives you the option of downloading and installing Cinnamon instead, then you have to download all of gnome 3/Cinnamon. Finally, if an ISO were to have Cinnamon as default, it might not work with your hardware (meaning you couldn't install), or if the ISO had MATE you wouldn't know if Cinnamon worked with your hardware. Having them together on one ISO, on the other hand, adds duplication of programs.
Dell XPS 15 l502x - Debian Testing 64-bit NetInst Xfce, SolydX 64-bit Debian Testing, SolydK 64-bit SolydXK Testing
Old Gateway Pentium 4 Desktop - Arch Linux 64-bit Xfce and SolydX 32-bit Sid