To preface - The PCs I work on at home and work are fairly decent - Intel i7 Haswell, 16GB of RAM, 256GB SSD, so it's not like my using it because my PC is old.

LXDE has some usability issues (such as random glitches, which seems to be mostly resolved since 13.04) over XFCE or other desktop environments but I find it quite a joy to use, especially as I work on a single monitor but have multiple workspaces which I can tab through using alt+scroll, or using scroll on the wallpaper - for example I have development (sublime/chrome) on workspace 1, emails on workspace 2, testing on workspace 3 (which runs in the background and is handy to run).

LXDE seems to be the only desktop environment that doesn't lag when I have quite a few things open and going off at once, but I need to re-experiment with XFCE now 13.10 has come out to give it a proper go.

Lubuntu is more lightweight, which will be important for older and lower powered machines (or if speed is a huge deal for you), but certain things might be a bit less polished. You would have to try it to see if it works for you. Xubuntu is a bit more full-featured and can feel somewhat like Gnome 2 did back in the day. But there's no reason you can't install both and try them on any Ubuntu machine, these are really just packages and defaults rather than completely different distros.