Mint 12 LXDE has a lighter and faster collection of programs and services so it makes better use of the Lightweight X11 Desktop Environment and it's a very complete LXDE distro so I uses a little more RAM than other LXDE distros but it's easy to remove what you don't need
so it becomes as fast and light as other LXDE distros.
It's possible to install LXDE on top of another Mint 12 edition but it takes a little more work to make it work just like Mint 12 LXDE. If speed and low RAM utilization is what you want LXDE is a good option but you don't have to wait for Mint LXDE, just install LXDE on top of Mint 12 (or Mint 13 LTS, LMDE, etc), change it to your taste
and remove all the Gnome3/Cinnamon stuff and programs/services you don't need.