They both suck but it's basically who would you rather have. A guy who can call a solid game but is a shit pitch framer, unathletic as hell (Collins) or have a guy who calls a below average game, great pitch framer, but a passed ball machine (Zavala).
Both have shit bats so that's a wash (Collins can take a walk, Zavala can run into one over the fence. Both K alot).