There are a lot of factors going towards it. Gio has had better past performance, better strikeout stuff, only left-handed starter, has pitched better lately, peripherals are close to career norms suggesting some bad luck at work especially with runners on, bigger contract, "playoff experience", etc...

Meanwhile Roark is by all accounts a team first guy who also has experience coming out of the bullpen. Also they have been priming the pump with that "matched his career high in innings" talk earlier in the month.

But that said, it is a decision that has to be made. The team is going to take a guy who is 5th in the league in RA9-WAR behind Kershaw, Cuueto, Wainwright, and Hamels and tell him that they'd prefer he throw less innings when games matter more.