HTC Dash / S620 / S621 (Excalibur)

The MDA is a full-fledged Pocket PC (Phone Edition) while the Dash is a Smartphone (which might be described as Pocket PC lite). If you need the additional functionality of the Pocket PC, get the MDA - if you don't, get the Dash since it is easier to carry size-wise. If you Google "Smartphone vs Pocket PC" and similar terms you'll find lots of info. Briefly, the Pocket PC has a touch screen and powerfull versions of MS Office apps while the smartphone has viewers only and no touchscreen.