It's great to have some numbers behind all these options.
As someone who switched to xfce around the time of Unity, but runs on a desktop with 16 GB of RAM, this article tells me that the differences here are not worth considering as a reason to pick one WM over another. But then again I'm using the heaviest option. :-)
I switched to Xfwm when Metacity started killing console beeps[0], then full[1] Xfce when Gnome 3 came around. As such, I can assure you that even with a mere 8 GB of RAM, there isn’t much of a difference – apart from functionality, that is.
[0] If you had a terminal window open, "echo \a\a\a\a" would not play anything if Metacity was running.
[1] Nautilus 2.32 is still sticking around because I rather dislike Xfce’s drawing of the desktop.
Slightly OT, but I think developers often over-optimize our technology choices. That is, we spend more time choosing between two options than it's really worth, because neither choice is bad. Nothing wrong with doing due diligence, but at some point it's fair to say "either option is okay" or "non-technical considerations are more important". I suspect this phenomenon shares something in common with language wars like Python vs. Ruby.
As someone who switched to xfce around the time of Unity, but runs on a desktop with 16 GB of RAM, this article tells me that the differences here are not worth considering as a reason to pick one WM over another. But then again I'm using the heaviest option. :-)