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.