Both are old technology. Although the Treo 680 is a new product, the Palm O/S is old and somewhat outdated. If its anything like the 3 Treo 600s and the 650 I used to have, then it will be junque. Prone to reboots several times a day, etc.
Similarly, the 8125 has been replaced by the 8525 which is a pretty good upgrade. Windows Mobile is an OK O/S, but still not as stable as BB or Symbian. I can't remember if the 8125 has WM 5 or not.
What do you want the phone to do for you? Tell us that so we can provide our input. Also I assume you will using Cingular a the carrier?