He can both of them if fit. Hell, he gave Djokovic a run for his money in New York with a bad wrist making it impossible for him to go for his BH (hence 0 BH winners), he definitely has a shot at a Slam if he can remain healthy. The two HC vs Nole this summer are hardly an indication as JMDP was playing with a bad left wrist all summer after the Olympics.
His biggest problem will remain Federer, who is a horrible matchup for him with his variety when he's playing well. Not to mention Delpo can't return Fed's serve for shit, makes Federer look like Isner on serve.
Delpo can only defeat Djokovic on grass where Djokovic defends terribly, Delpo cant hit backhand winners against Djokovic because Djokovic's defense is far too good to allow Delpo's predictable backhand to get past him. I have seen all of their matches and the matchupissue is all there in the open, Delpo is too predictable when attacking Djokovic to cause any problems.
He and Berdych (and Soderling) can only defeat Djokovic on grass. Tsonga can do more damage because of his superior serve.
Murray might be even worse, Murray got that variation to add pressure on Delpo.