Value wise Grey will most likely be worth more, but Thin and Thick would be somewhat comparable.
People that care about the stamps and consistent in a full set would go with Thick stamps, since non-Holo Thin stamps are close to non-existent.
But in reality, most collectors don’t even know about the differences in stamps, and if they know, most don’t care about them.
Personally I like the look of the Thin stamps more, and in my Pikachu collection I try to get every possible 1st ed. stamp and even distinguish 2D, 3D, and back-imprinted stamps.
But for a 1st edition Charizard I think most people would be glad to own one regardless of the stamp, so I don’t think Thin/Thick stamps will have any difference in value, even though the Thick stamps are harder to find.
Greetz,
Quuador