Hernandez had a 4.4 WAR (FG) season last year.
If that's not good enough, what the heck is?
This is a guy who hit .309 in AAA at age 23, got blocked from starting the next season, had a decent rookie season in 2015 and showed substantial improvement last year.
He's probably not as good defensively as he showed last year, but that doesn't mean he'll regress all the way to "average" either, and his hitting could improve further. His base running may also improve with experience.
I have no problem with a 3-4 WAR 2B who turns 27 in May, I'd say we'd be in great shape if we had guys performing like that at six other starting positions.
Kingery really fell off when he went to Reading, that's a red flag to me, a 22 year old top prospect shouldn't struggle that much, both his BB% (3.0) and K% (21.7) collapsed at Reading to unacceptable levels for a "singles" hitter. SSS and it may just be "adjustment lag," but still, a black mark.
Valentin's lowest BB% was 8.2% in 2014, last year he was 9.8% in both AA and AAA ball. He's also consistently hit with more power than Kingery.
If you look at the Fansgraph spring 2016 current and potential ranking of tools, Kingery's only edge is speed, Valentin has the edge on arm strength, which allows him to function as a utility infielder, and that makes him more valuable than Kingery (who is a 2B only prospect).
This is why this season is key for both prospects, Valentin is auditioning for a utility IF spot, Kingery to stay in the organization (if Hernandez has another good season, Kingery will be traded or left unprotected, depending on how well he plays, since he has little value off the bench).