best Smartphone for corporate plans

between question and discussion:
We are currently using Symbian with Nokia E7x phones in our office, a couple of users have Windows Mobile on HTC phones. Due to mergers and reorganizations around the world, we want to standardize and implement one solution for everybody. Up to now I am leaning towards Blackberry, but I found realy hard to be objective with all the criteria needed to be considered.
I would appreciate your comments on what are you looking for when choosing a smartphone plan, pros and cons between the existing solutions (I am considering Palm, Blackberry, Windows Mobile, Symbian, even Apple, although I would wait to see how it works for others at corporate level – heard HSBC down-under are migrating :-))