The main difference is in the desktop environment (DE) and not in the installed applications.
(The main applications like Libre office suite and Firefox and Chromium internet browsers act ok in LXDE on computers that otherwise cannot cope with Ubuntu/Unity).
People say 'Ubuntu' to refer to the DE of the main Ubuntu release which uses Unity as DE.
Ubuntu-proper = Ubuntu OS + Gnome/Unity DE
Kubuntu = Ubuntu OS + KDE
Xubuntu = Ubuntu OS + Xfce DE
Lubuntu = Ubuntu OS + LXDE
The difference in resources used by Unity compared to LXDE is huge (from the point of view of my 1GB RAM PC).
LXDE/Lubuntu is the lightest, but it takes some more customization to make it look stylish (like this). (It's easy in fact.)
Xfce/Xubuntu is a bit heavier, but still light, KDE/Kubuntu is close to Gnome/Unity in the use of resources, the later being the heaviest.
For example I have a computer that is sluggish in KDE and Unity but works ok in Xfce/Xubuntu and is perfect in LXDE/Lubuntu.
You can install all the different DE that make up the Ubuntu family, try them, and then stuck to the one that best meets your needs and your computer's specs, but having them together is not very useful on the long run.
I have used Lubuntu on top of Xubuntu with no problems and do not think would be any problem to put it on top of Ubuntu-proper. I fact a fresh install of Lubuntu would mean lesser stuff being installed initially, that you could add afterwards anyway (like Libre Office for example). It all depends on your needs.