My input: Both are very good and both does the same things in different intensities.

O2 XDA II: Get this is you will need more functions of PDA than a phone.
PROS: Its a very good PDA. Its a pocket PC. It can do everything (most) that your desktop or laptop can do like view and edit Word, Excel and Powerpoint Files and more. Lots of Third Party Software available.

CONS: Its quite big. Difficult to put in pocket if going to a party or any place where your PDA is hardly used. Relatively harder to use as a normal phone (text).

P900: Get this if you need more Phone features than PDA features.

PROS: Quite stable (compared to P800). Nice looking. Not as big and bulky as most pure PDAs. Easy to use. Very good phone features. Decent PDA features.

CONS: Lacks in PDA features (compared to O2 XDAII).

In my humble opinion, and if I can only have 1, I would choose p900 because I use more phone features than PDA features. And besides the PDA features of P900 are sufficient for regular my needs.

I have both P900 and 02XDAII and I find myself leaving the XDA in the car most of the time and only bring it down when I have "serious PDA work" to do. Can't live without both though.

I look at the P900 is a comfortable condominium and XDA is a luxurious mansion and both come with their respective maintenance costs or in this case user interface challenges.

Hope this helps.