Ubuntu: Xfce or Lxde?

Which one is faster?

Obviously LXDE. It is even lighter than XFCE. LXDE uses about 110 mb of ram when idle while XFCE is a little more bloated, taking about 200 mb (based on my experience)

Which is easier to use?

XFCE. It have more GUIs while LXDE requires you to modify configuration files in your own in some cases.

More configurable?

Both are configurable, but it depends on how much you want to play with it. It is less configurable than KDE or GNOME, though.

Can I install them?

Via the software center everything is easier. But I recommend you use lubuntu-desktop for LXDE or xubntu-desktop for XFCE. (Since they've prepared most of the stuff for you)

With my experiences, Xfce (or Xubuntu), as an older DE, is more polished, provides more "built-in" features, has nicer artwork, and a larger community.

LXDE (or Lubuntu) on the other hand is a newer DE, not that polished as Xfce, it takes several apps from Ubuntu and Xubuntu instead of having it's own equivalent (for example notifications, power manager, disk manager, and window manager (Openbox)). But Lubuntu is much faster than Xubuntu, mainly because it uses lighter applications and hasn't got all the eye-candy (effects, shadows, compositoning).

They both are full-featured. If you are installing Linux on someone elses hardware (friend, parents), who are new to Linux, go with Xubuntu. It's more user friendly, and has a nice settings center. However, if you are installing it on your own hardware, try Lubuntu. Not as nice (although you can fix that), needs a little tinkering, but the speed will fascinate you.

I am was on Lubuntu (actually GNOME on Arch now, hehe), mainly because of the speed, but I just installed Xubuntu into my cousin's computer yesterday, who hasn't seen a Linux yet, because it's easier to operate.

Please note that I was talking about Lubuntu 14.04.1 and Xubuntu 14.04.1.

However, with 15.04 (maybe 14.10 but that's unlikely) They still haven't swapped it but it looks like the plans are after 16.04, Lubuntu will swap the now abandoned LXDE with LXQt, on which 2 teams (LXDE and RazorQt teams) will be working on, and will get much more polish and be more like Xfce, and Xfce will be ported to Gtk3, which is not exactly lightweight, so the difference will be much larger between Lubuntu and Xubuntu.