I prefer Herrera, but last year he was bad enough that you can't assume he's an upgrade over McCutchen or Harper in CF... and those two guys would only play there if Quinn was hurt. If Quinn were to be hurt long-term you'd have to go looking for another CF.
I would argue that if they sign Harper, they have not truly improved the OF defense, except in LF. where anyone would be better and McCutchen is basically slumming (but is not good enough to play the other two spots). Herrera and Harper are both coming off negative seasons. It is broke, that's why they're trying to fix it!
It's just like any theoretical trade - what best addresses short and long-term needs, both on the team and in a trade return. If you give up on Herrera it has to be because he gets you more in return than Williams does. But that's not necessariy the case for now.