Palm of course ;>
I am a Palm Fan for several years (Palm III, Palm T2, now T|X) and very appreciate it. For GTD, I use a Wiki application name NoteStudio with the addition of a Plugin name CreateGTDActionList that detect, from each pages in the Wiki (for me a page could be interpreted as a project), the actions that reside there and create a report, regrouping all action tag together (@action, @errands, @home,@pc, etc).
For me, one of the more important criteria was the possibility to synch between the PDA and my XP machine. 95% of the data entry is being made on the XP and the Palm is used mainly for consultation.
The reson I have mentionned about NoteStudio is that I think your decision shouldn't be only about the device, but the software you are intending to use. Even if you have the best device, your GTD system will be affected if the kind of application you would like can't run on it.
Knowing what kind of software each people are using on Palm or DELL/WinCE will probably help you deciding what kind of device would benifit you.
To the addition of NoteStudio (my main GTD XP and Palm application), I also use KeySuite, which synchronize outlook with the Palm, adding several features to the default outlook synchronisation utility that come with several Palm device.
Hope this could be useful for you. Have a good shopping.