<<...I noticed most slates only have a XGA screen...>>
Kurohio: Over the course of time, XGA has established itself as a good "standard" resolution for slates. Companies like Motion have tried SXGA+ screens, and I believe that you can still get them, but newer offerings have returned to XGA. The resolution is not bad; I think it is actually quite compatible with the size of typical inking (it doesn't affect note-taking) and that is one reason why it is such a common resolution.
Regarding WXGA, the primary difference is the screen aspect ratio. Many of these screens are 12.1", so they simply end up being taller but narrower than a 12.1" XGA screen. I'm glossing over a few details, here, that others may regard as important, but in my opinion, there is littlepractical difference between XGA and WXGA, although the latter is certainly nicer if one of your primary uses is to watch videos / movies...
<<...can anyone give me any feedback on how inking on the ST5112 is?..>>
...The inking is very good. The ST5112 uses the Wacom digitizer and pen; both are very mature both electronically and mechanically, and they provide the kind of inking experience that one would hope for and expect.
<<...what about Dual Capacitative...>>
...I own and use both and there's no question in my mind that N-trig's DuoSense digitizer represents the next wave in tablet development. Their touch experience is very harmonious with their pen experience and the whole effect is very seductive. But it is (essentially) only available on the Dell XT, a convertible. The Motion LE1700wt was the only slate expected to have capacitive touch and it has since been withdrawn (and is unlikely to be marketed).
Right now, the promise of touch is largely diminished by the lack of any touch-based applications to exploit this new capability. Thus, you lose little by choosing a conventional active digitizer for now. If touch takes off, there will be plenty of time over the next couple of years to upgrade your slate and join the revolution in progress!