Using AT&T is in no way a problem. It just helps to know because if you're using AT&T you'll definitely need a phone that's quad band (with the 850MHz band in particular) and you might want 3G (T-Mo's US 3G network really isn't built-out yet).
All 3 phones you mention are nice, but the Samsung Omnia and Touch Diamond lack US 3G which is a drawback if you want to access the Net and email when out of range of a WiFi hotspot. Not that they can't do it but EDGE is a lot slower. Rumor has it that a US 3G version of the Diamond might hit at the end of the month-- worth waiting for vs. the current Euro-only 3G version. But the Diamond does not have a microSD card slot (though it does have 4 gigs of internal flash memory).
My personal favorite at the moment is the Omnia, though it really hurts that it doesn't have 3G (I use the Net and data more than I talk on the phone!).