Leyland grows thicker. If you plant them close enough, your hedge can tolerate a dead tree without any visible gaps.
Where as if you have a dead GT, you will definitely see a dead tree or a gap.
Leyland are very tough but they don't like wet feet. Same as the GT.
I see alot of people from my area would let their Leyland screen grow out and top off to a desire height.
To be sure if they can survive the extreme weather from your area, I suggest to drive around town and check out how your neighbor's trees. That would be a sure bet.