Well the obvious choice is Volquez. I would keep Volquez. Even if you think Cueto is better, Volquez's value is very high.
That being said...
It's not as far off as
Johnny Cueto: 8.17 K/9, 3.52 BB/9, 38.6% grounders
Edison Volquez: 9.46 K/9, 4.27 BB/9, 46.3% grounders
Volquez has more Ks and grounders, while Cueto walks less guys. I'll take Volquez's K/GB combo, especially in Cincy, but Cueto also has plenty of upside. In 07, he put up insane K/BB numbers through 3 levels and his HR numbers weren't bad.
Cueto wont be even close to 4.81 ERA next year, probably more like a 4. I'd put Volquez in the mid-high 3s.
So the answer is keep Volquez, but Cueto is a solid sleeper.
And BTW, Dusty could destroy them both.