2 Inch torsion key and 2 inch block?

Your payload can't change, it is what it is from the factory.
Keys are an option but you could get the same effect by just cranking up the torsion bar adjusters

You can get 2" with some room to adjust still sometimes 2.5"
Both keys and cranking achieve lift the same way, by increasing tension in the front end. The only advantage to keys is that it allows you to keep the stock adjustability

Add a leaf is more reliable because blocks can cause axle wrap and wheel hop which leads to worse things
At 2" there really isn't a difference, when umyou start using larger blocks with bigger heavier tires it becomes more of a problem