dont bother getting both. i highly recommend you just get ubuntu, then on top of it use apt to install kde and xfce, perhaps even fluxbox and e17 and then that will be your guinea pig setup. you can try out all the desktops for yourself and see which one you like most. here is a quick outline of the above so you know if its even worht trying or not:
KDE -- the most "windows-looking" of the bunch. lots of programs (nice ones too) are built off the kdelibs library or with the qt toolkit. it is pretty hard core in terms of system requirements though.
Gnome -- another "heavy hitter" but takes a slightly different approach to design and function. uses the gtk toolkit instead of QT. from my experience its a bit lighter (in terms of ram use) than kde, but still quite hefty.
XFCE -- if i am not mistaken started out as a "lite" build of gnome. still uses gtk and all that nice stuff but takes a hell lot less ram and incorporates some minimalist elements such as a light-weight file browser (xffm or thunar instead of nautilus).
Fluxbox -- the uber lightweight totally minimalistic desktop. you get a small tiny taskbar and thats about it. takes very little ram and has that "nerdy" appeal to it. most configuration is done through editing little text files rather than through a gui interface like in the above 3.
E17 --- the latest build of the Enlightenment desktop. Extremely lightweight and rather minimalistic (not as much as fluxbox thogh) takes very little ram (i wanna say 17mb...compared to about 200 of gnome) , takes again manual configuration but has some SWEET looking eye candy.