Honestly what's up with the first base eligibility being so invaluable? Their current permanent positions are at C -- that's what matters a good hitter eligible at the catcher position.
Santana gets the nod because he already possess the ability to knock it out of the park more easily.
the odds of Posey being valuable as a catcher are probably much higher than his odds of being valuable as a first basemen given supply. I don't see how the first base eligibility really makes a difference other than assuring owners that Posey should get at bats since he can play at either position. As pointed out, though, once Santana is up he should be playing so I don't think it makes a difference.
As for which one, who knows. I like Posey, but I like Santana's potential fantasy value more since he appears to have more power.
When you are a man, sometimes you wear stretchy pants in your room...is for fun.