XFCE is much more lightweight and will consume much less resources then what KDE would, but it really is a personal preference. IMO, the differences between Gnome, XFCE, and KDE are pretty much blurred at this point in time.
Never really tried loading KDE or Gnome from a Xubuntu install (or visa versa), so I'm not certain how difficult it is to switch desktops with the same install. I know opensuse, you can load these and choose what you want to use at the GDM screen when they're installed, so that may be a simpler option for you. Personally, I'd probably go with either Gnome until you get familiar enough with things before trying XFCE.
I don't know there's a "best" distro for a laptop. Ideally, I'd identify what kind of wireless card you have and use that as your deterinent for what you use as I would think everything else should be supported to some extent.