I can't speak for Debian, but for my money, XFCE is more integrated, LXDE isn't. SSH agents aren't installed, for example, and aren't trivial to manually install. Looking for LXDE help has you palmed off to Openbox as often as not. I think LXDE looks better - XFCE looks coarse and ungainly, but there is better integration. Still not perfect, but if it's moved to be the debian default, there will be more eyes on it.

Is the XFCE/xubuntu community more active? I suppose that plays a big role on how much ongoing development activity there is...

The Xfce forum is helpful, and they do happily answer Xubuntu questions. (Frequently the answer is "yeah, pull that in from GNOME" or "that hasn't been written yet, please do!" but that's fair enough in many cases ;-)

I've been a huge fan of LXDE ever since I went to make the clock show seconds, and it literally told me to look up man 3 strftime. THAT is how you get rid of non-serious users.

In my experience, XFCE has been prettier (for me at least, using the Xubuntu theme).

You might as well use a standalone window manager + various apps if you need something more "lightweight" than xfce.