I think it's important to note that the previous summit votes have often placed faction interests at odds. For that reason I'm inclined to put the white sky here, since we can afford to take a hit (and it's very possible their bonuses will help mitigate a downside. Furthermore, both parties will be the primary people trading and potentially traveling across the border. Having the both highly invested means more wealth, resources, and renown for our fief.
Emerald Seas nobility are also the ones we most want to cozy up with post-summit, so getting them high will make our future projects easier. By contrast, the polar theocracy is more a force we need to keep appeased. Obviously bigger number better, but we aren't really aiming for that. We already have a positive point of favorability and a roll bonus as leeway if it's needed. I also don't want to potentially risk that if a matter is between them and the Emerald Seas nobility.
TL;DR: White sky can both take a hit, and benefits more from being higher score. Whether the number goes up or down, it's better than Polar Theocracy.
[X] White Sky Confederation
[X] Emerald Seas Nobility