Cinnamon does have more active developers than MATE, but there is still a lot to do in Cinnamon and each new release brings further completion and perfection of the vision the developers have for Cinnamon. For example 1.6 saw Nemo introduced as the new file manager, and 1.8 will introduce Cinnamon control center, for all your settings and preferences, and Cinnamon screensaver. (Some screenshots and info in the "Latest development" section here: http://blog.linuxmint.com/?p=2318
Cinnamon is an exiting project, attracting developers interested in working with modern technologies and helping to further shape Cinnamon.
MATE is based on GNOME 2, so it has a very large and stable base. The developers are very much active there also, and passionate about what they are doing. Remaining bugs from GNOME 2 are being fixed (though there are less and less) and the team is working on introducing new features and adapting MATE also to modern technologies.
(You can explore the MATE project's GitHub
page to see what is happening, for example here are the recent commits to Caja showing fixes and work being done: https://github.com/mate-desktop/mate-fi ... its/master
. Likewise you can explore the Linux Mint project's GitHub
page, for example here are the recent commits to Cinnamon where you see a lot of "pull request" with changes from contributing developers being included: https://github.com/linuxmint/Cinnamon/commits/master
The Linux Mint developers also play a role in the MATE project, and the MATE developers also help out on the Linux Mint project. With Cinnamon being the most popular desktop environment amongst Linux Mint users and MATE the second most popular, and with both desktops being adopted by more and more Linux distros, I think you need not currently worry about "which is more likely to continue". Pick the one that works best for you
As to your first impression, it is known that VirtualBox's mouse can be glitchy on Linux Mint 14 Cinnamon (do make sure you have enabled 3D acceleration in your virtual machine's settings, and share as much video memory as possible). As passerby suggested, try it from a DVD or USB stick instead to get a good impression on your real hardware. The glitchiness of VirtualBox's mouse can be worked around a bit by disabling the hotspot in Cinnamon Settings (you can trigger the hotspot also with Ctrl+Alt+Up). Another user, but I can't find the post right now, had a script with a few settings that you could run to fix VirtualBox's mouse. Though experiences may differ, on my hardware I found MATE uses more resources and is slower than Cinnamon. So test them out also on real hardware, and pick the one you like best for the things that matter to you