I say Jeremy Hefner.
He had a 3.66 FIP last year and a crazy low walk rate(1.73 BB/9) in 93.2 IP. His xFIP is a little higher at 4.21, but that is still perfectly acceptable for a 5th starter. His K/BB was 3.44 too which is great.
His 5.09 ERA looks like a fluke because he didn't allow many homers and his LOB% was very low. His BABIP looked higher than it probably should be(.319)
He also gets a decent amount of GBs(43.8%) so he seems like a worthy option.
Bill James projects a 3.87 ERA and 3.62 FIP for him in 2013 which would be tremendous.
I'd rather focus on the bullpen again, and it will be easier to demote Hefner to the pen than a FA starter for when Wheeler comes up. I think it makes sense.
I'm sure people will want Mejia to get a shot, but honestly his command doesn't seem good enough to be a starter. It's a huge issue for him that needs serious adjustment.