What's the difference in size between the expansion pad at EU and Kidzone at USO? Depending on the ideas being developed, that would probably play a large part in which park a Pokemon land would go.
Pokemon is a huge, worldwide phenomenon, easily as popular and well known as Mario or Mickey (and that's fact, not just a fan speak), with huge potential for ticket and merchandise sales, as well as its own land. I can't imagine either company in the partnership wanting to do less than their best in bringing it to one of the parks, so which park has the space and potential to benefit the most? There's a lot of ride and interactive experience potential in Pokemon (and not just playful kiddie stuff, but really strong dark ride potential featuring the canonically creepy Ghost types and their backstories--even more so than a Luigi's Mansion dark ride--which would be amazing in SNW instead of putting Pokemon in there) they wouldn't want to waste by not doing a full land. It will be insanely popular in either park, so if they're going full-out, which park is the better option. If EU, I feel it would deserve its own land separate from SNW.