Pros:
Palm = incredibly stable, as long as the firmware was done right, wide range of apps available, very easy to navigate with just the 4-way key (they have done a lot of work on this over many revisions of treo, you can seriously do *everything* on a palm treo without the stylus)
Windows = better for media, easy synch with all windows apps (esp. if you use Outlook for calendar/contacts/email), built in word/excel/powerpoint that work well and don't take a ton of storage
Cons:
Palm = it's an ancient OS, a lot of stuff that it does is kind of lame. i think they finally did bundle an mp3 player in the 700P (the 650s do not have a GOOD free mp3 player, they come with realplayer and that's it). i haven't seen a 700p yet so i don't know what other changes have been made to the OS.
Windows = it's windows. it tends to crash more from what i've read. the navigation isn't quite as streamlined - have to go through more menus to get the same stuff done. by default, it doesn't quit programs when you start other ones, so it's fairly easy to use up the program RAM on the phone and cause it to lock or perform very slowly. there are ways around this, but it's a pain in general.
Disclaimer: I currently use a Treo 650, and work is moving to Verizon and at the same time, requiring us to go to windows phones if we want to keep e-mail/calendar sync (we currently use GoodLink and are dropping it). I'm very worried that i won't be able to find a windows phone i can tolerate... my experiences are based around what i've seen on Windows mobile 2003 PDAs, which the smartphones are similar to but not exactly like.