After watching countless games on TV last year (and several in person), here are my thoughts on the catching platoon.
Hannigan was so much better at getting his pitcher strikes that were borderline or off the plate. He gives the umpire that extra good luck of "it was right on the black" and that is invaluable for your pitcher.
I think Mesaraco has more potential as an offensive player but lacks the discipline of Hannigan. Hannigan's OBP skills is important in the 8 hole if our pitchers can successful get the bunt down (which wasn't a strength last year). Mesaraco projects as a better 6 or 7 hole hitter but didn't live up to hitting expectations last year.
I hope Mesaraco can grow into that "receiver" that Hannigan already is because I think his bat will come around.
At this point, however, its 3 games of Hannigan to 2 games of Mesaraco for me. Mes needs to catch the pitchers with the best control and let Hannigan "win" some pitches for the other 3.