I'd take either guy really. In keeper leagues, I always err on the side of the position player since SP can be so volitile, just look at Greinke. Johnson is a hell of a pitcher, but he was also listed on the Verducci report prior to this season and has had injury troubles in the past. He's no sure thing, and while no one is, Votto and Agon are much safer bets.. If we were to assume that everyone stays healthy and on the same teams (I'll get to this in a second), then I'd actually argue that all 3 guys are pretty much equal in terms of value. However, if AGon is traded, especially to a team like the Sawks, he could have a huge jump in value. That's for you to decide if you want to take that chance.
As for me, gun to my head, I'd go with Votto.
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