The Blue Jays signed Sanchez for a good reason; catcher depth.
In case of injury, the Blue Jays would be forced to rely on Josh Thole. The only problem is if he or Russell Martin get injured, or if he needed a day off (catchers tend to require one more frequently than other positions), who would replace him?
Not really a matter of ‘who is better’ right now but, rather, who could fill in the backup slot in the case of injury to either Thole or Martin. Of course, this will draw comparisons between Thole and Sanchez, and I think you could make a strong case for either side.