I think I would rather sign Christian Ponder. And not because he already knows Norv's system. But because he would be more successful if used like Dak. Give him RPO (run-pass options) and he can use his athletic ability to make plays. He is also more accurate on roll outs, just not a great drop back passer. As a back up to Dak, he is my preference, do not have to change plays or game plans.
Open up playbook when Tony get's back. No other back up out there is going to do what Tony does, but a simplified playbook that makes them read run-pass would allow more options at back up QB.