I am inclined to say Djokovic is better than Murray, as he beat Nadal at Wimbledon whereas Murray has lost to Nadal three straight times, and would have lost to Nadal in 2011, 2012, and 2013 if they had met. If Djokovic met Nadal in these tournaments, the result would have been a coin toss as Djokovic has a fighting chance.
Although Murray heads the grass h2h against Djokovic, in this case I think their h2h against Nadal tells the more important story, so Djokovic is better than Murray on grass, imo.
What do you think?
Murray's head-to-head against Djokovic on grass is a product of small sample size. It's not sustainable and has no basis for further generalization.
Murray didn't need to lose to Nadal hypothetically in 2011 at Wimbledon as you put it; he really did lose in four sets in the semi despite playing very well by his limited standards.
Djokovic and Murray each have a lone Wimbledon title. Djokovic faced a rather tough draw in 2011 including Tsonga in the semi and Tomic playing the best tournament of his short career, and Baghdatis in earlier rounds. Nadal was having his best Wimbledon ever, with no early round brinksmanship as is his norm, and Djokovic still crushed him decisively in the final. While it may have been, in part, due to his winning streak and supreme confidence, it does demonstrate what Djokovic is capable of on grass.
Murray, on the other hand, barely breezed through the easiest slam winner's draw since Nadal's USO in 2010, almost falling to a hitherto invisible Verdasco and struggling against nobody Janowicz who miraculously made the semi. The Djokovic he faced in the final was barely able to move under the distinctly un-English hot sun (the roof should have been closed) after coming through a resurgent Del Potro playing his best tennis since 2009.
If you could rank their respective Wimbledon titles in terms of legitimacy out of 10, Djokovic's would be about a 7.5 (deduction for not having to play Federer in the semi), and Murray's a 2 if I'm being charitable--which I always am with Murray.
(The Olympics, as far as tennis goes, is more of a patriotic circus than a real tour event, so there is no point in bringing up Murray beating a decrepit old man and an indifferent Djokovic there.)