I own both and prefer the Grey....Altough the Green is more iconic and I admit on pics he looks better,when you got them side by side the grey looks more impressive, the eyes for exemple makes a better contrast on the grey.The pics we have seen except maybe the ones by occulum dont do the grey one justice...He looks 100x better in reality.I've never seen pics of the grey where you see the awesome paint wash,but when you got it in front of you, its there....Its a great discussion piece also.
For the casual fan,I think the green one is the one to get.But for someone who is really a fan of the character,the grey one is special,its a great representation of a moment in the history of the character.
If I had to choose between them, regardless of the ES, I'd kept the grey.
About the grail factor, yes its a small ES, but, in a couple months from now you'll see the green one getting more and more valuable also....since a lot of people want that version...I see the green one getting to 1 K eventually....
So, I think, arguably, they are both grails...its just a matter of which one do you prefer...