FWIW, I believe that the generally accepted Exchange support is best acheived using the 3rd party app called "Touchdown". There are some people having a few issues here and there but for the most part its compatible with most people's Exhange versions. Just search the forums for "Touchdown" to see where the issues lie.
As far as OS vs. OS, I believe WinMo is more "laggy" and "problematic" on touch screen devices, in some situations. I have WinMo 6.1 on a Motorola Q (more like a BBerry Bold in form), and its very stable, even after several "mods". There is a big WinMo modding community and its easier to "root" since one just needs a registry editor, which is readily available for a few bucks.
However I think that Android is the momentum play as Win 7's fate is still up in the air. Android has the stronger device line up as well, and given its market share, there will be more 3rd party apps/accessories.