having used both OSs (not on the Treo 700 however), I'd have to say Palm if you want a real tool, Windows if you want a nice looking phone. Windows mobile acts far to much like its big brother, and requries frequent resets, and can act stupid sometimes for no apparent reason.
Palm is better, less resets (not 0, but less), IMO easier to use since everything is far simpler,
However the downside to Palm IMO is that it doesn't come with much built in, document syncronization, multimedia, MP3, sound recorder, a decent clock, etc aren't built into the OS the way that they are on windows mobile so you have to go otu and get them yourself (they are getting better, I think the 700p and the 650 come with most of those things on the ROM, not made by Palm however), and I still think Activesync (windows) is better than Hot Sync...
but despite this, I'd still go Palm....reliability>*