I'd take Greinke/Gallardo.
A summer watching a bad Reds' team, is still a pretty good summer.
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.