I'd take Greinke/Gallardo.
Zero chance the Reds miss the playoffs!
If we're taking contract situation into account, then I'd prefer Latos and Cueto. If we're just talking about ability, I'd take Greinke and Gallardo since I don't really trust Cueto to reproduce last year's results and I think he'll continue to have some durability issues due to his small frame.
But consider last year. You would not have even considered this a serious question with Volquez/Wood starting games 1 and 2.