Regarding GNOME vs. KDE, why don't you create LiveDVDs or LivePenDrives of both editions, and try them both out for a week or two before actually installing? That way you'll get a feel for them both and be able to make your own comparison.
I use KDE on my main laptop, and Xfce on a couple of other laptops. Xfce is another good DE to consider, actually, and it is very usable these days, albeit not quite as configurable as KDE. If you want to know more about KDE, you may be interested to read the following couple of blog posts I wrote regarding my own KDE installation: A guided tour of my KDE 4.8.4 desktop (Part 1)
& A guided tour of my KDE 4.8.4 desktop (Part 2)
, which are basically still applicable to the latest release of KDE (4.12.0).
Regarding your question about setting up partitions, the SL Installer can do that for you if you want, or you can set up all the partitions manually instead if you want (that's what I prefer to do, actually, as it seems to make installation more likely to succeed on the machines on which I have installed SL). You can do the latter before running the installer or within the installer, as you wish. I usually do it before, using GParted.