Is there any indication the S3 will actually have Super AMOLED+ or is that just wishful thinking? GSII phones should be getting updates to Android 4.0 very soon, so that can't be the difference. US versions of S3 won't have quad-core processors, so that can't be the difference.
The only thing I can think of is they just won't release it in the US until the 5250 Exynos chip is ready and/or Super AMOLED+ HD exists. And that might be a while...