It's unlikely that both will return next season so which one are you thinking is the better option? Although neither will command a huge salary I'd like to see us be able to have 3 decent QB's on the roster and actually have some depth if Batch needs to play and then gets hurt.
By going with one veteran and either Smith or Johnson you might be able to save a few bucks against the cap too.
I would say go with Batch and then let Troy Smith and Jerrod Johnson fight it out for other backup spot. I just can't see bringing back Leftwich right now after he spent the last 2 years on IR. If he were healthy or gave you the inclination that he could stay healthy then maybe he'd be the better option. Mainly because he's younger if nothing else.
I'm totally fine with Batch backing up Ben for another year or 2. If Smith does make the final cut maybe they can use him in a slash type role...