Koskinen has at least been a solid goalie in Europe (KHL + WHC), and had pretty good stretches in the NHL last year. He may not be a great starting goalie in the NHL, but at least he's got experience from bunch of different stages, and he's 6'7" which is an advantage in some situations.
Korpisalo though.. He's been a very meh backup in the NHL, never held star status anywhere and absolutely crapped the bed for Finland in WHC last time he played there (almost 4 GAA in 6 appearances). And perhaps most importantly, he's broken 45 pro hockey game mark ONCE in a season since turning pro. That was the 15-16 season, his first in NA. He had a great stretch run with the Jackets when they were already out of the playoffs and in my opinion he's been riding that ever since.
Im not trying to bash him before the season even starts, it's a huge year for him. But let's be real, his body of work is weak and now he's being relied on to be the starter, much like Koskinen last year. At least Koskinen has that experience already.
Maybe Korpi does serviceable job next year and a guy like Anderson doesn't have it anymore, so im not saying Korpi should "win" this poll in a landslide.. but Korpisalo is not better than Koskinen right now.