The 3 main PDA OS's are
- Windows Mobile
- Palm OS
- Symbian
I will compare Windows Mobile (WM) and Palm for you.
WM is a very powerful OS, however it is very inconvient and complicated. While WM can integrate well with your PC, the OS is slow sometimes, freezes up more often than Palm, it is very unorganized and not as clean looking as Palm, and the only way to really close and exit an application is by going into the memory menu and manually closing the app. With Windows Mobile, you can browse the web with Pocket Internet Explorer, or any 3rd party web browser you have downloaded. You can download apps from Handango or Mobihand that are very useful, like Bible software, GPS software, and many others. While there are currently many apps for Palm as well as Windows, this phone expert predicts a downfall in the production of Palm apps. This is because the number of PDAs using Palm is decreasing rapidly. Most companies are using Windows Mobile in their products. Unless things turn around, Palm is not going to be popular app platform not too far in the future. You can e-mail people, both with Corporate servers via Exchange and POP3 and IMAP personal email accounts. You can basically do anything on your Windows Mobile PDA that you can do on your XP PC. You can view and stream media like videos and music.
Palm is a good OS too. It is very stable, easy to use, and equally powerful. However, like I stated before, the amount of apps will decrease. Also, Palm doesn't integrate as well as WM. There is a pack on the Palm devices called Documents To Go, which allows you to create, edit, and view MS office documents. And of cource you can sync your outlook data using HotSync. Just keep in mind that WM WILL integrate better with a Windows PC. It's pretty obvious that a Windows product works well with another windows product.
I would like to know what PDA you would be planning on buying. I just saw your other question about the TyTN. I'll answer you right here. It's a great PDA. The only PDA on Cingular currently that has WCDMA HSPDA 3G data. The only other one is the BlackJack but that is only a smartphone and not nearly as useful as a full PDA. It has killer reception. The data speeds are insane. It is very stable. The battery life is OK, but you gotta expect that from a HSDPA PDA. For more info, you can Y! IM me by going into my profile and clicking IM.