Racket technology has not improved in the longest time. The best players' rackets today are still widely considered as the PC600, Pro staff moulds, POG etc. Players use paint jobs and those with a keen eye can spot them from a mile away. String technology improved vastly in the late 90's with the likes of Luxilon developing etc but still, there's no better all-round string than gut.
Goran loved playing with gut and used Babolat All Season pretty much throughout his career. There's no way that today he's going to jump to some synthetic like Big Banger, RPM Blast or whatever. Considering he's 41, had umpteen shoulder problems and no longer has close to the power in his legs as a result of his age tells me that he's either being fooled by tampered speed guns or he's just trying to be smart. I've lost count at the amount of times I've seen past pros bombing down serves at exhibitions and on the Blackrock tour that look barely 115mph and they're registering 125mph+.
Regarding Muzza and Nole you can argue either way. Murray's more natural on it and moves better but Nole has the biggest title.
Murray- 1 Wimbledon final, 1 Olympic title and 2 Queens
Djokovic- 1 Wimbledon victory, 1 Halle final and 1 Queens final
I guess both are valid.
If I have to choose one of these to play for my life would choose Murray any day. Still like you said, Nole has Wimby title, Murray doesn't