Interesting dilemma, to say the least. Although both titles share the same product and name, both are also exceptional in their own right. I'm sure this must happen from time to time, though, what with the vast number of parodies made over the years.
Despite the fact that I already own Neil's version - and Dave's version would make a nice addition to my collection, as well - a certain respect must prevail between the various artists and their respective creations. I suppose this is unfortunate for collectors always eager to collect more, however crossing lines would inevitably lead to not only bad feelings, but also anarchy, a vast free-for-all in which anything - names, details, etc. - would be up for grabs and inevitably undermine the integrity of WP as a whole (which we've kind of already seen on occasion this year.)