Yes, Xfce will be much faster than Unity. That's not the distro that makes the difference though, that is the desktop environment. When you compare it like that, Xfce is probably more customizable than Unity also.
Linux Mint has a bigger repository than Ubuntu, but you could add the Linux Mint repository to Ubuntu also if you really wanted to (http://packages.linuxmint.com/list.php?release=Nadia
). The Ubuntu Software Center sells software also, and if you want that on Linux Mint you will have to install Ubuntu Software Center and make some tweaks to be able to buy software from it on Linux Mint. But if you would be doing that for games, you can install and use the Steam client or the Desura client for gaming on Linux Mint also
Stability wise, both distros will be running the same kernel so that won't make a difference. Again this goes to the desktop environment mostly. Xfce is known to be developed for a stable base, with paced development of new features. Unity on the other hand has always been developed rapidly, gaining new features and redesigns each release. Not saying that makes Unity less stable, but it is certainly a major risk for it.
As for security, again with both running the same kernel there is but small difference. Linux Mint does enable Samba
by default, to facilitate interoperability with other computers and devices in your househould. You can easily disable that if you want.