Smart Hulk took longer and had more pain whilst wielding the Infinity Stones in the Infinity Gauntlet because he was trying to bring Nat back and undo the previous snap by Thanos. Bringing Nat back is, however, impossible and so he was essentially fighting the Stones before he just gave in and reversed the Snap.
Smart Hulk: You know, I tried. When I had the gauntlet, the stones, I really tried to bring her back.
However, Tony just removed the Thanos, his Children and his armies from 2014 so had less to do.
As we can see the affects of the snap on Smart Hulk were less than they were on Tony who:
Dies from it
Whereas Smart Hulk merely has his arm hurt.
It’s also worth noting that Smart Hulk has superhuman healing abilities and so has a higher chance of surviving the Stone’s powers as he would be more resilient that Stark who is a normal, albeit highly intelligent, human.
Smart Hulk was "made for this" as his own exposure to gamma radiation, both in the initial accident and later on to combine Hulk and Banner into Smart Hulk, matches that of the Stones' radiation. So it seems he always had a greater chance of surviving the snap than Tony.
Smart Hulk: We don't. But the radiation's mostly gamma. It's like... uh... I was made for this.
Lastly, Tony's armour also takes away some of the energy of the Stones and the Snap off of himself, as you can see the power coursing across it and the damage to it afterwards from the Snap. So whilst he is hurt more than Smart Hulk, he survives longer than he should have done because the suit saved him a bit.