And the ironic part is that his market perception is suffering for it more than benefiting from it. He had one excellent year, yes, and also four other years ranging from average to very good.
Obvoiusly, how you see him relative to Sandoval is going to depend on how much importance you place on defense. Headley has a 114 career wRC+. Sandoval, 122. So clearly Sandoval is the better hitter. But if you factor in defense, the advanced metrics say that Pablo is essentially a league average 3B (career DRS 0, UZR/150 2.2). Headley has a career DRS of 29 (in about 4.5 years' worth of innings) and a UZR/150 of 10.8. So Headley has been about 6 to 8 runs a year better on defense than Sandoval. I think you'd have a hard time making a case that the offensive difference is bigger than that.
So I think Headley is, at worst, as good a player as Sandoval. It might make more sense to go longer with Sandoval, but not by as much as the age difference between them, due both to the general durability questions around Panda's weight, and also the related question of whether he'll need to move to a position where his skills have less value. If you're willing to give Panda 6 years, you should be willing to give Headley 4. And the AAV of the Headley deal should be, if anything, a bit higher. So any logic that produces a 6/110 deal for Panda should justify at least a 4/72 deal for Headley. The fact that nobody is talking about anything remotely that big for Headley means that he is much the better value of the two, and therefore is the guy we should be going after.