Does LXDE use Qt?

LXDE is moving toward Qt in 2013 and tries to co-operate with Razor-qt, another lightweight desktop environment.

Which is better XFCE or MATE?

Although it misses a few features and its development is slower than Cinnamon’s, MATE runs faster, uses less resources and is more stable than Cinnamon. Xfce is a lightweight desktop environment. It doesn’t support as many features as Cinnamon or MATE, but it’s extremely stable and very light on resource usage.