We've never been to either of the ones you mentioned. But they have really good chocolate shakes at The Plaza Restaurant at the Magic Kingdom! My DH has one every year we have lunch or dinner there.
Mark me down as another fan of the Primetime Cafe - as I, too, am a PB&J Shake junkie (although admittedly, I have yet to try B&C - though I'm going to make a point of getting there within the next 4 mo (and yes, that is an entirely arbitrary deadline))...
My brother always orders the chocolate milkshake at Beaches and Cream - without the sprinkles! When we ate there in January, the waitress saw me eating the cherry from his shake, and brought me a little bowl of about a dozen more!