never tried kde plasma, i tried kde 3, and it was surprisingly good, then i tried kde 4 and it was terrible. i imagine it's improved since then though. (it was real early kde4) is plasma light weight?
KDE4 is KDE plasma. (I think officially KDE 4 plasma workspaces)
KDE4 was terrible when first released but it has now evolved into a very nice and mature desktop.
I'm currently using release 4.9.5
Gnome3 seems to be going through the same pains as KDE4 did in its early days.