In my experience with PDA's over the years. That being:
Palm Pilot (Original) -> Apple Newton (Still use it.) -> Casio Cassiopeia E-11 -> Palm V -> Compaq iPaq 3535 -> Compaq iPaq 37xx -> HP Jornada 568 -> [To be determined this fall]
And supporting about 30 Palm's in the office I work I think both the Pocket PC and the Palm have their pros and cons. Both are solid platforms. Both do the basic PIM functions relatively well (In Palm's case very well.) But that is where the similarities end. Palm is a great organizer. When you start to get into more complex functionality and capabilities you start to run into problems. Pocket PC started off with a more robust OS out of the gate.
This resulted in issues with system stability in Pocket PC 2000. (We are currently on Pocket PC 2003 SE.) Since that time the underlying OS has been upgraded from Windows CE 3.x to 4.x and enough tweaks have occurred to make it relatively stable. (I liken it to still being Windows 9x but its a Windows 98 version. Not as good as a Windows 2K or XP release but not bad.) So why be concerned with a more robust OS? Basically it can do quite a few things that Palm can't. Pocket PC (Officially called Windows Mobile but I refuse to give up that name damn it!) 2003 SE has built in landscape support. Which works out well for e-mail reading, movie watching, and web browsing. Built in support is slick as heck. You don't have to worry about 3rd party apps and since its built into the OS software developers can take advantage of this mode for the device. So developers like softmaker can make their software natively support landscape:
Why is the screenshot so big? That leads me into point two. The latest release of the device has native VGA support out of the box in some devices. OEM's are having problems getting their hands on VGA screens. Now first thought would be to think that ICK. Too small. I don't want to be squinting at the screen. Under normal circumstances yes. That would be a concern. However MS has implemented, similar to Sony when they went to a higher res then Palm's 160 x 160 screens, pixel doubling and font resizing so in the end it looks like this:
QVGA (Standard Pocket PC's):
VGA:
If you can't tell I'm in love with VGA screens.
In the last two years Palm has done a remarkable job at brining their hardware up to par. They are just about there with a few issues. Palms typically are now pricier then their Pocket PC counterparts when you look at the features provided. I think this will change in the next 18 months or so but for now that where we are at.
Beyond the hardware the PPC OS acts like a true OS in some ways. Part of this is why Palm is faster then the Pocket PC. Its a double-edged sword. The core of Palm OS is not a true file system but a set of databases and the method of how Palm programs function is drasticly different then how the Pocket PC works which again is more like a miniPC then anything else.) This allows Palm and its core PIM functions to be DAMN fast. But on the flip side of the coin it's a serious limitation to what can be done with and in the OS. Example would be creating a mounted drive with the device. This can be done in Palm OS but Sony did this with some creative acrobatics via software. In addition to that Palm OS still doesn't have the ability to multitask. So lets say you want to work on a quick and dirty e-mail and you want to cut and paste info from a word doc into the e-mail. In Palm OS you can't have both apps open at the same time.
Palm again has done some creative tweaking to the OS to allow some apps to be open at the same time but I believe that it's only the core PIM and MP3 player apps. Supposedly Cobol, Palm's next major iteration of the OS, will rectify this situation but I'm not going to hold my breath.
Actually I'm expecting a whole heck of a lot from Cobol because if it doesn't deliver Palm is going to be in trouble. But that's another topic.
I personally think both platforms have their place. Palm for basic PIM with some additional app capabilities. Pocket PC for more computer like functionality. (For the love of god you can open, view and edit CAD files on a PPC! And someone, can't remember the company, is working on video capture on a Pocket PC

) General rule of thumb. If you can do it on a PC you can do it on a Pocket PC. Games, movies, word, excel, powerpoint, acrobat, web, IM, photo manipulation, CAD, network monitoring tools, remote control, GPS, video out to projectors for presentations, consumer IR to remote control average consumer appliances. Heck there is a sled for the iPaq for providing EKG's for paramedics. (Actually there are a ton of solutions for the vertical market.)
It really depends on what you want to do. I personally am either going for:
HP 47xx.
-624MHz XScale CPU
-4" VGA screen
-183MB of storage space. (Keeping in mind that a certain % is required to run programs and such.)
-WIFI and BlueTooth
-SD and CF card slots for storage and accessory expansion
-touchpad navigation
or
The Dell Axim X50. [You guys think you are nuts about hardware. Pocket PC sites have been dredging up as much info they can get on this model. So far I've seen at least a dozen leaked drawings of what it's going to look like] So far most of it came from the FCC site.
The model is going to sport a
-624MHz XScale CPU
-3.7" VGA screen
-128MB of storage space. (Keeping in mind that a certain % is required to run programs and such.)
-WIFI and BlueTooth
that was figured out because of this FCC picture:
In both cases the devices are going to be spendy but I've gotten into the grove of one new PDA every two years so plunking 500-650 on a new system doesn't hurt as much. As for your original question about compatibility. MS doesn't have a version of ActiveStink for OS X which, trust me, is a good thing. If there is anything that should be complained about on the Pocket PC its their craptastic sync software. Activesuck isn't fit to hold Palm hotsync's jock strap. Its slow, its buggy, its the Windows ME of the sync software world. Fortunately several other vendors have stepped up to the plate with syncing software that integrates into Apple's iSync flawlessly.
The first of these solutions is
http://www.pocketmac.net/ that runs, I think, around $45. Spendy. The second is
http://www.markspace.com/missingsync_pocketpc.html
Which is just as spendy at $50. So if you are looking for compatibility its there but it will cost you some extra $$ to get it and its not a full solution since installing apps is somewhat a PITA and 3rd party conduits like Pocket Quicken doesn't works since they are based around Activesync oops activesuck.
Palm compatibility is there for now. They have full compatibility out of the box with hotsync and Palm Desktop. As I said before last I heard Palm is dropping support for OS X when they move to Cobol. Again there is
http://www.markspace.com/missingsync_palmos.html which directly link to iSync. So all in all its pretty much a tossup.