You have to decide what you really want out of your second baseman.
Do you want a player who is primarily an offensive player, who may be only adequate defensively?
Or, do you want a player who is solid defensively, who may be OK, but not great at the plate?
Cesar Hernandez doesn't quite fit either of these "types." At the plate, he's shown a respectable OBP (.331 careerr, .339 in 2015), but little power - Isolated Power of .064. Defensively, the more advanced stats say he's consistently below average at second base.
So he's below average defensively; what kind of offense would offset that? In my mind, a .340 OBP with little or no power isn't enough. If his OBP were .380, I'd be content with his offensive contributions. If his defense were average, I'd be content with .340. But his defense is well below average, and his offensive contributions are limited. He's not dreadful, but 2B is a position where improvement is clearly possible, without having to find another potential HOF-caliber player.
Freddy Galvis? Offensively, he's behind Hernendez; defensively, at 2B, he's clearly better - advanced defensive metrics say he's a net positive (Hernandez is a negative) at the position. At the plate, he's had trouble maintaining a .650 OPS, and that's notwithstanding that he has some power (IP of .111); his OBP is lousy.
Now, what I would like to see at 2B (assuming I can't have a potential HOF'er there!) is a guy whose defense, at a minimum, is neutral, and whose offensive contribution is OK (OPS around .700 or so) or better. I don't much want a 2B whose defense is negative (Hernandez); I also don't much want one whose OPS is stuck at .650 or less (Galvis).
