The carrier will release the device based on their specifications, software, etc. You can get info on the basic differences in processor speed/memory/features @ HTC.com. This is research you can do on your own- it's pretty well laid out.
Which one is 'best' will be a subjective issue, dependent upon what your requirements are in a handset, and perhaps whether or not a carrier has neutered anything the phone has under the hood. For example, the ATT Tilt is somewhat less responsive than if you load a third party (non-carrier) ROM. YMMV. You might check out the HTC wiki site, and also howardforums.com for more specifics.
Whats is the difference between the HTC touch pro/ fuze and the diamond pro? And which one is the best