Ignoring the actual handset make and model, which phone handset OS would you buy right now regardless of what you have currently?
My thoughts on the matter are that the actual phone OS is having a bigger and bigger impact on peoples decisions to buy a handset. A good example is the HTC Desire and SE X10 - the HTC has very little downsides compared to the SE X10, the main gripe being the camera resolution.
However the SE has an older Android OS and its own custom UI doesnt seem as nice as HTC sense.
Nokias seem to be barking up the wrong tree releasing more and more phones on Symbian, whilst efficient it doesnt have the featureset or developers behind it - ala Blackberry App World, IPhone App Store etc
Blackberry's are the thing to have for instant email on the move with good support from RIM themselves with apps.