I'd take Alonso. He's a better hitter and in no danger of losing playing time. Gattis is a decent short term option at catcher, but with McCann healthy and Heyward back in a couple weeks, he'll be on the bench 5 days every week.
"If you can't spot the sucker in your first half hour at the table, then you are the sucker."
-Mike McD, Rounders