More desirable? Probably not, except by hardcore collectors like myself. Most people couldn’t care less about Grey/Thick/Thin, and the majority of the collectors don’t even know these differences exist.
More valuable? Perhaps slightly, but since 1st edition Charizards are already worth so much money, I doubt the difference in price-offers is noticeable.
Having said that, if I had a Grey or Thin stamp Charizard, I would definitely charge more than a regular Thick 1st edition Charizard.
Really? I thought it was the other way around. I personally like the look of Thin stamps more myself, and I was under the impression the majority that know the differences thought the same. And you only mean Thick is more desirable than Thin? Or also rarer?
For non-holos Thin are without a doubt rarer than Thick, but I thought for holos this was the case as well (90% thick, 10% thin or something like that). I think @charchu said something along those lines in another thread (not sure though).
There’s a few reasons thick has a higher demand. Thick stamps were first, making thick stamps the first edition of the first edition. You’re not going to find a complete thin stamp set, but you can have a complete thick stamp set; a lot of collectors like uniformity. Thick stamps are slightly rarer for holos; extreme opposite for non holos. (No clue on how much rarer or any ratio distribution. I would guess 60-40) @garyis2000 probably can give you a better ratio spread.
For those that don’t know here the story for the existence of thick/thin stamps:
Wizards produced a bunch of cards with thick stamps, when they got done they realized they miscalculated their ratio of holo to non holo cards for packs. To correct the issue, Wizards sent another round of holo cards to get stamped. Between the time it took to get the second stamp the pressure used to stamp the cards was changed, creating thin stamps.