IR at St Andrews genuinely seen as the best in the country, that's including Oxbridge, Kings is good but not seen as good. The Will and Kate comment is a bit ridiculous- doesn't have anything to do with the uni.
With the two what's more important is what you prefer in terms of a place to live for 3/4 years. They are so different and I assume you've got a preference about being a city or a town by the sea.