XFCE provides precious little performance improvement compared to GNOME (at least in my experience), but with the loss of considerable functionality and a hundred mildly obnoxious quirks that combine to make it not at all worth my while.
XFCE is gotten more bloated in recent releases. However we have started to notice not as wide of a cap between the performance of XFCE and GNOME/ KDE than in the past.
LXDE starts instantly, it is blazing fast and is very highly configurable. It also has a clean, uncluttered interface that gets out of your way. PCManFM (the original, and central, component of LXDE) is hands down the best file manager ever written, lxpanel is both lightweight and very functional, and of course there is the power and flexibility of OpenBox underneath. What's not to like?
free memory after startxfce4: 1,789,552
free memory after startlxde: 1,804,456
XFCE isn't as lightweight as it used to be and in fact operates slower than gnome in many setups and distributions. So for the purpose of being lightweight/smaller footprint I will have to say LXDE, but you asked which one is better and that all depends on what you want. XFCE is plenty more functional or rather feature rich. So its a matter of compromise between function and smaller footprint.