Im afraid my approach to the question is pretty far off the beaten path and even selfish. I blend play and writing together - and Cyrodiil, as the center of Tamriel, makes a fine anchor as far as games go. I had big plans for my story longer term that involved Skyrim. Then Bethesda announced that TES V would be in Skyrim. Not their fault, but in broke my heart and shattered the plans I had for that province because, sure enough, Bethesdas Skyrim does not match my own by a long shot. I now am working on plans that involve storylines in both Elsweyr and Valenwood and am concerned about history repeating itself with TES VI. The rich variety of Khajiit in Elsweyr (as mentioned above) and the complete absurdity of The Green Pact give me great confidence that my concept for these provinces would never align with Bethesdas. Therefore, I voted Summerset Isle, simply because it is more isolated from Cyrodiil and I have no plans for it.
Well, everyone has an opinion and thats mine.
*runs for cover*