Basically what it comes down to is the use of the OS. There are Blackberry and Windows Mobile, both will do the same things in the end, but the OS's run a bit diffrently. I've only had experiences with Windows Mobile and it was slooooow, I would imagine the Blackberry OS would be a little more responsive. And pretty much any unlocked PDA will work with ATT although there might be a process to go through to get all the network settings working correctly, but some phones have an option to do it automatically so it's also something to look into.