Chris Perez or Sandoval. Sandoval will get 3B eligibility 5 games into the season, though it might take a while to get him C eligible. Perez should get first crack at the closer job and has pretty good stuff. It's really close, but I'd go with Perez to save the extra $2.
I'm 50-50 on Perez or Sandoval but with it this tight I'm gonna say save the $2 and take a flyer that Perez ends up closing. A $6 potential closer in an NL only league who could post respectable ratios with solid k totals doesn't seem like a terrible option to me. You may even be able to get Sandoval back for a similar price depending on what others catchers are kept or the bidding on the others (and the level of patience people have in your league who would be drafting him to play catcher). He certainly isn't the worst 3B once he gains eligibility given the possibility he'll hit in a "juicy" part of the Giants order (if there is such a thing... and I am a Giants fan, so I feel no need to sugar coat this ). Ultimately it was close, but like I said, I think you should go Perez. Good luck!