Isner's forehand is actually excellent, especially inside out. It's his backhand that is such a liability.
What keeps Nole's forehand down is that on grass, it sucks. It breaks down easily, against Berdych it was shocking.
Low balls bother him. He's great at hitting hard on high balls but on low balls he can't get power behind the forehand as easily. His forehands to low slices are very average generally.
On topic, Djokovic has the better forehand. More power and can control play with it.
Murray has a better pass, especially crosscourt.
Consistency, I'd say they're about even. Murray might be slightly more consistent but on the other hand he is a slightly more defensive player than Djokovic.
Don't really know what Typhoon/Whatever is rambling about Murray's forehand being deeper. If you had to choose one attribute that separates Djokovic's game from other pros and what makes him a top player it's the deepness of his shots. Rarely do his shots drop inside the service line. Murray's forehand can drop short and the topspin is far less aggressive than Nole's making it a shot easier to attack.