You can get a COMPLETELY decked out Pocket PC for about $350 from Dell. My Axim x30 624mzh arrived today. I LOVE IT! This is after having HP's top-of-the-line hx4705 which I had paid over $600 for.
If you don't need a completely decked out pocket pc, dell still has very good ones for as low as $200.
I don't have experience with Palm stuff. I'm a Microsoft Specialist and am onlly interested in MS OS's. I also have the Motorolla mpx220 smartphone that runs windows which I also recommend.
Concerning implementing GTD with ppc or palm, it has more to do with the software that you can run or install on it that matters. I'd recommend ListPro or TreNotes (those are the top 2 list managers for the ppc)
I have a Palm Tungsten C and I love it. I like the qwerty keyboard -- good if you type in a lot of stuff. I use it "out of the box" except that I added Documents to Go, since the memo capacity is small and I like to carry around larger documents/notes.
It really depends on your exact requirements. If you are planning on doing most of your text entry on a computer, and only need a simple GTD system, then any Palm (preferably high res) combined with the standard palm apps would be fine.
I know a lot of our customers using NoteStudio for their GTD system have a Treo 600/650. Having a phone and keyboard always available makes this quite a good system. Makes the @phone list interesting when you are actually looking at the list from a phone!
I presonally have a Zire 71, which works well for me, although I'm thinking of getting something with a bigger screen, maybe a tapwave zodiac, which more and more of our customers are using.
I just upgraded to a Palm Zire72 from an old Tungsten T. I love it. It's great for work and play. With a memory expansion card I have been able to load it great programs. But I also love having a mp3 player and camera all in the same tidy package.
I think you need to understand what any of the two systems can do and can not do in regards to GTD. Correct me if I'm wrong (GTD Folks) but to this day I think there is still no clean solution to getting GTD "Projects" from Outlook to a Plam or pocket PC. This is due to a problem with how Outlook handles the fields compared to Palm/Pocket PC.
You also have to remember that what you are asking is subjective. People from either group are evangelistic about their devices. As Dan put it, you really need to know what you want to do with the device first. Getting a device soley on the premise of GTD would probably not be wise.
I have gone from my first PDA (actually before they were called PDA's) a Sharp Organizer, then a Palm Pilot (before the Pilot pen company sued), then after reviewing several Palms, decided on a Pocket PC. Needless to say I love my iPAQ. I have the software tweaked just right for all of my chores. Word and Excel are marginally OK, but if you want to retain 100% formatting then you have to go with SoftMaker's TextMaker and PlanMaker. I can have my photos with me (digital wallet for pictures of my kid), digital music player for plane trips, and they just released new software that will allow you to record a full DVD movie to a memory card for playback.
Again, if you work with a lot of "Projects" that field (Project field vs. Category field)does not come over from Outlook.