I think they are about equal in terms of upside, but Ledezma has an injury history, and thus has been very erratic over his pro career. I think he will settle into the bullpen ultimately given his durability problems. Tata seems to be a relatively safe pick (as far as pitchers go anyways) to be a future #3 or #4 caliber starter, but don't expect tons of strikeouts. Does not have the durability problems so far that Ledezma has. I don't think either one has huge potential in your average 12 team league, but if I had to pick between the two for a deeper league (or an AL only league) I would go for Tata given that Ledezma carries much greater risk.
Also the Tigers will probably have to part with young pitching if they want to upgrade the offense and add another hitter, and Ledezma and Tata would probably be among the names that are talked about.
Hopefully someone else here can add something if more needs to be said.