I don't know what to think of Myers to be honest. There's a wide range of things he could turn into. If he settles into a .270/.340/.450 line with ~23 bombs (Jay Bruce with more average, less power), then he's not a special type of asset. That's a very possible scenario IMO.
But he does have big tools and could be a real impact bat. Profar I don't love, but he could be an above-average middle infielder for a decade. There's value in that.
I'd probably take the Posey side.