I'm in the same boat. I don't own either book and am not looking to buy either book. But if I was, I would be looking for a UF4, because it's got a much bigger upside.
Generally, I can understand being a little wary of just looking at the last sale to determine a value, because sometimes there are outlier sales, and maybe there just aren't a lot of eyes on some sales. But these are both very visible books, and we have two instances of pretty much the perfect comparison of the books; sales in the same grade closing very near to each other on the same selling platform. And in both cases, UF4 is on top. And the comparable recent average shows UF4 is on top. If you want to go with something like a 5-year moving average, then I guess 667 wins out, but honestly who is going to care about that, except someone with an agenda to make a particular book the winner?