As for stamina, I would think Djokovic is better.
The most objective reason is that I believe being lighter must help in the long run : when I saw both of them live, it was obvious Djokovic was less "fine" technically (Murray has a great touch of the ball imo) but much "finer" physically, he's smaller with thin (and beautiful
) legs while Murray is tall with thick legs.
Then if I look at the past I remember some good showings of stamina from both :
- I remember a Djokovic who had great stamina in his early years, esp I remember Wimbledon 2007 but also many good 5-setters in his early years.
- Murray proved a good stamina in the US Open 2008 where he had several tough early rounds, also winning from being led like against Melzer. And he still had some energy left to end the tournament very well against Wawrinka and Nadal (I don't remember who he faced in QF).
But since then they have not looked as good, it seems. And we have so many pictures of them whining during their matches, looking exhausted ...
But apart from being light, the second reason why I think Djokovic has better stamina is that if both of them have been used to complaining during their matches, but I've always been astonished how Djokovic can forget bad points to play a good point next. I remember I had been shocked by that against Baghdatis in Wimbledon 2007. Many of his matches have been made with constant ups-and-downs but he often managed to get a little bit more of "ups" than downs, esp in clutch moments.
On the other side, when Murray starts whining, he seems to get systematically negative, esp in the matches which proved tougher than what he thought in the beginning. And he gives up. Watching Djokovic against Troicki then Murray against Wawrinka in the US Open was a good example in my mind. Murray looks like a child in these matches while Djokovic has proved very mature already in his early years.
Besides, yes, Djokovic has some health problems, esp in hot conditions, but Murray is not perfect either : I remember how he complained in Cinci last year.
For the return of serve, I also think Djokovic is better.