Comes down to cardio to me. JDS has shown a pretty amazing ability to avoid the ground. Cain will have a hell of a time taking him down and avoiding his striking.
I say, standing up JDS would have the edge, but Cain is a prodigy and always improving. Cain will work the clinch and cage, his pace and stamina could very well be the difference in a 5 round His striking is at the very least, enough to stay alive and work his wrestling, and at the very most, enough to beat JDS.
Cain is probably the more well rounded fighter, but Dos Santos strengths match up well with him. I still don't think its enough though. Cain should be favourited.