Nice and solid game!
The level design is good but i think that it didn't generate that many moments in which the player has to chose which way is better to complete the level. Most of the times I completed the levels with Ben and when i couldn't, Zombie was pretty easy to complete too.
What i'm trying to say is that it never felt like a bad thing to die or lose since, even if i had limited health, i could just finish the level with the other character.