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.