It's tough. It's tempting to say Djoker. He's a year younger than Nadal, has had far less injuries and has a style of game that shouldn't be as quickly ravaged by age. Nadal only has clay as Djoker is the superior player on the other surfaces at this point.
On the other hand, tennis drug testing is a joke, so Rafa could easily pull a Manny Pacquaio and stay on the gear to still have the athleticism of a 20-year-old. Rafa's French dominance also gives him more tournaments that are in the bag. If he wins at least two more of them, which you would think he will, then that means he effectively has 4 in the bank on Djokovic. There's no guarantee that either player will pull a Federer, and could very quickly fall off a cliff.
So I'm going with Rafa. I think the two-slam buffer and the likelihood of future French victories will carry him across the line.