HTC Dash / S620 / S621 (Excalibur)

It really depends on what you want to use it for. For example, while both the mda and the dash have a 1.3 megapixel camera, and even though the mda has flash and the dash doesn't, pictures taken on the dash are notably better, even in the dark without the flash. They both use mostly the same programs, mostly. The MDA however, has excel, word, and powerpoint applications, while the dash only has viewers. I would like to emphasize though that with both phones, there is a plethora of third party software available at sites like handango that allow you to make your phone what you want. I personally have the dash and am in love with it. It's more phone size, and unlike the mda, you can use it with one hand. I don't really know what else yo...
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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.

ive had my mda for a while, i like it but am thinking of going to the pearl. my fiance loves his mda and doesnt want to switch to anything that he cant use his mda for. he likes the touch screen the best and we have found lots of diffrent hacks and programs for it.
im a student in college and found a nice planner for classes and different games to play while bored, and also a voice recorder. i put in all my appointments and its so easy to be reminded. but like the dash mine tends to light up randomly. im sure there are many similarities and its all up to you to figure which will be the most useful