In few words, Palm OS 5 types wins on multimedia.

Note they are running ARM architecture, not older Palm OS 4 and below. Those older types are using Motorola Dragonball architecture (incidentally a cousin to the classic Mac architecture) ARM is now THE architecture for both Pocket PC and Palm. XScale is an Intel variant of this architecture that takes account for PDAs' need to overclock/underclock in favour for performance/battery life. Avoid Palm OS 4 types if multimedia is what you have in mind.

I have both a Tungsten T3 and iPaq 2215, they are of both running the same CPU (Intel XScale PXA255 400 MHz) and I have TCPMP for both of them. Regardless the fact T3 has larger and finer screen the Palm machine edges out often at times although sometimes Pocket PC comes out in favour, Microsoft Windows Media format is one clear example.

If you have a need to play Microsoft WMV or WMA, Pocket PC with TCPMP is the best combination yet, Pocket Windows Media Player simply not efficient and where as a background MP3 was choppy to the point of unlistenable it was running smoothly within TCPMP for Pocket PC.

Due to the fact Palm OS presets power management with XScale to auto it would be wise to keep an eye on the CPU speed when playing files approaching desktop-caliber (yes either Palm or Pocket can play desktop files just fine, albeit only encoded a lower quality files) luckily TCPMP
 and few other software can give you that kind of information (speed, type, even revisions of the CPU). This is true for higher end Pocket PC too. There are XScale adjuster programs available in both Palm (PXAClocker) and Pocket (XCPUScalar) that can overwrite any OS behavior.

I am unable to give comparison on office tasks and PIM tasks. There are plenty of such comparisons already; although I have smoother experience with Palm OS machines over Pocket PC. For the record, I use Windows desktop computers, I have not touched a Mac in years.