Another interesting question
So it's close if we come up with hypotheticals instead of look at reality? Okaaay. Nadal made five consecutive (times played) Wimbledon finals winning it twice and dethroning the greatest there. Djokovic isn't even close to that regardless of how many Masters he vultures now other players are declining. Of course Grassdal sucks now and loses to nobodies, it'll happen to Djokovic on clay at some point too.It's pretty close. Of course Nadal has a better record on Wimbledon than Djokovic in Roland Garros, but if they both had played in the 90's with Sampras, Ivanisevic, Krajicek, etc. on grass and it being a lot faster, while in RG you had not-that-great players winning it, it could have been a lot different.
If we judge level of play, it's very close, results, obviously Nadal.