Windows Mobile Professional

In regards to hardware, they are the same device. They are both manufactured by a Taiwanese company called High Tech Corp. (HTC) and codenamed Wizard. The 8125 is the WIZA100 and the MDA is the WIZA200. Only difference is the form factor. Some say that the 8125's keyboard is better, i've got no complaints with the MDA's keyboard. Now as for software, each one will come with its own carrier specific customizations. Whatever customizations the provider has added, they can be customized to your needs. I own a I-Mate K-Jam purchased overseas(before the US release of the MDA or 8125) and i'm running it with my personally customized rom on the cingular network. Head over to www.xda-developers.com and they'd got a bunch of information that you could use about customizations and problems & fixes that others have faced with their devices. I'd say it boils down to which provider you'd prefer. Ofcourse if you switch over to cingular and sign a 2-year contract you'd get that 100 dollar off the price. Also Blackberry connect is available on both devices.