My only worry with selling it is not being able to find another. The money would be good, but money ain't everything. Scarcity is much more valuable to me.
That is indeed a question that you need to decide yourself. If the numbers are definitely true and there is no chance of more popping up and the rarity of the proof compared to the commonality of the artwork is worth that much to you then I would say hold onto it. I have done the same more than once myself. At the same time for me a huge portion of what I enjoy about this hobby is the hunt.
The one thing that you can guarantee though is it will not be worth this much in a matter of months so if you do decide to sell now is the time to do it.
...Another thought... You could always just send it to me and you would always now where one is at least.
