Its a tough call since Byron seems to have hit his stride.
In the context to build a team i don't think it's a tough call to be honest. If you have to chose one this year to win in playoffs then yeah a tough call.
But if you have to chose one for the next 5 years to contend every year then you have to go with the safer bet and it is Gallagher imo. He's more proven and also younger. There's injuries concern but that should not stop you from taking him over Byron.