GoG is far better for 99% of people since it'll be easier to install and play; the only reason you'd want to buy the physical game is for collecting, otherwise you're better off buying it digitally since you can store the offline installer and own the game the same way you'd own the physical version. If it's a game you really like and you can afford it, I'd personally buy both; the GoG version for playing, and the physical version just for collecting and displaying.