I would stick with ISDN.
First of all a robbed bit T1 is much more difficult to trouble shoot and get fixed if you ever do have a problem. I can not begin to tell you how many times we have argued with COs in an attempt to fix a problem that was with there equipment and probably would not have occoured, or would have been easier to pin point if it had been ISDN. Also ISDN is Extremely faster when you are talking in terms of predictive dialers, also the response from the CO being digital instead of an audio signal means that your system will register a response (busy, No answer, disconnected, etc…) more accuratley.