hi, Xubuntu vs Lubuntu is personally would say for a lighter experience go with Lubuntu, if you want to have a little bit more performance and some more speed, then go with Xubuntu. This how i am personally bench marking my experience!
I have a single dell inspiron B120, with 2 GB of ddr2 ram @ 400Mhz speed,
a Celeron CPU @ 1.4 Ghz, integrated video, integrated sound, and 250 GB HDD for Gnome and 40 GB HDD for Lxde.
when I goto Youtube depending on the video in Gnome the video can get robotic, and with Lxde in firefox it is good, but with Google Chrome it is great! with all that said, this is all over a wireless g connection, i not using a n network adapter, the router can goto N, but i do fine with g adapter. Always have and i can watch hulu video, youtube movies and other video, tv.com, crackle video, once in a while i feel i should up grade my adapter to N but not yet. I do believe since I once did have Xubuntu installed for about 6 months before taking over the 40GB HDD from Xubuntu to Lunbuntu that hands down Lubuntu is lighter, but i am also finding less eye candy, as far as themes and so on.
I hope this helps.