In Avengers: Endgame, when Hulk wore all the Infinity Stones he was instantly getting major injury before he even snapped just by putting on the gauntlet. Whereas Thanos in Avengers: Infinity War was able to wield and use all 6 (not all of them at once) without injury (until he snapped). Is this because the nano gauntlet and the IW gauntlet are different?
Because Thanos is stronger than the Hulk.
The screenwriters Christopher Markus and Stephen McFeely have said in previous interviews that Thanos is simply stronger than Hulk, that is why he could beat him so easily at the start of Avengers: Infinity War.
I think he could. I may be speaking out of turn, but I think Thanos could kick the Hulk's ass without the (Power Stone).
We want to announce Thanos as the biggest villain in the MCU, so he takes out the previous reigning champion. And by defeating Hulk relatively easily and savagely, hopefully there's a sense of dread over the course of the rest of the movie for anyone else who comes up against him.
On top of this Hulk is struggling a lot longer with the Gauntlet on because he is fighting the Infinity Stones to try and bring Nat back but it wouldn't let him. I.e. his snap was more than just a quick "kill 50% of all life" but a back and forth fight.
Bruce: You know, I tried. When I had the gauntlet, the stones, I really tried to bring her back.
His snap attempt is instant, as in he puts the Gauntlet on and instantly starts trying to bring Nat back and do the snap. Had he left it on for a bit before attempting the snap the effects probably would have died down as we see when Thanos wears it.
Of course the two Gauntlets are different as well, made of different materials and designs. One would speculate that whilst the Infinity Gauntlet is probably made of a stronger material (Uru most likely but it's never mentioned), the Stark Gauntlet is an improved-upon design. Note that Tony, Professor Hulk and Rocket oversee its creation and they all incredibly clever. They've all also come into contact with The Infinity Stones before, and Tony and Banner also made extended analysis of the Mind Stone.
They also probably have the Infinity Gauntlet to look at after Thanos' death so can reverse engineer that. It's likely that the Stark Gauntlet could even be a better version than the original.
1. Different Gauntlet composition
The gauntlet in Infinity War (Thanos-Gauntlet) was forged by Eitri, the Dwarf King using
Uru (the metal used to make Mjolnir).
The material used to make the Iron-Gauntlet on the other hand isn’t specified but it’s speculated to be made using
Vibranium, the strongest metal on Earth
2. Hulk vs. Thanos
As seen during the fight between the two in the beginning of Infinity War, and as mentioned by MovieHub here, Thanos is way more powerful (both mentally and physically) than The Hulk.