I've had zero issues with McClelland Virginia Flakes degrading after opening a tin. I let the tin sit for 2 to 3 weeks after opening it to breathe. Then take my time smoking it, usually a couple of months to finish the tin.
They are not the same at all. Peterson Flake is a straight medium Virginia Flake. OGS is A Virginia with a small amount of Perique added. Both are quite good but different.