I'm coming at things from the other direction--for years I preferred window managers like E15 and E16; Blackbox, Fluxbox, and Openbox; and tiling window managers like DWM, Xmonad, and Awesome. I did have flings with both Gnome and KDE at various times, but they both always seemed far too bulky and monolithic.
So I like Xfce because it's a compromise for an old man like me who's not as enamored with constantly tweaking .conf files as I used to be. It's integrated enough, but not too much. I can make it work well and look good through a simple, centralized settings interface, and it plays well with the Gnome/KDE apps I like to use. It's a little slower than I'm used to, but it's still good. It's lighter on resources as you say, but that's not really an issue, since my desktop is a beast. However, I've been using Xfce to good effect on my gutless old netbook.
I also like the Openbox setup on CrunchBang, though for some reason I don't like LXDE, which also uses Openbox. They each have their own feeling, I guess.
Since you're interested in these comparisons, I recommend that you create a few partitions on your main (or a spare) machine, and install a few different Linux distros with a variety of DEs and WMs. One of the beauties of Linux is the range of choices it gives you. It can be a lot of fun exploring what you can do with 20GB and an installation disk, and it's not risky because you can always wipe out these partitions and try something else. You can also install several different DEs and WMs on a single Linux partition, and switch between them.
I used to run eight or so different distros on my main machine, but like I said, I'm getting old, and I only have a few these days.
When it comes to deciding what to use, there's no substitute for trying things out for yourself, because it's all very subjective.