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.
I have grown to really love Lubuntu because it strikes the perfect balance for older laptops. With a few tweaks, can look as beautiful though basic, and it uses RAM and CPU sparingly.
To me, it's a different distro because it only comes with the lightweight choices. (I found it very time-consuming to strip down Ubuntu, compared to tweaking Lubuntu upward).
The only minor criticism I could have is that it's a lot of work to make a Lubuntu theme from scratch, because of the many components to theme.
