I've only had the Carvel ice cream cake, never been to a location. Based on the chocolate on chocolate with the cookie crunch in the middle alone, Carvel wins. They got it right with the ratio of chocolate to sweet.
Coldstone is just chunky ass milkshakes you'd eat with your fingers if they didn't give you a spoon, don't kid yourself -- so that's like comparing bonafide albums to "Remastered Best Of" Retrospectives. Apples to orange-flavored sherbet.
It's hard to get bad ice cream, but once you've had something other than Baskin & Robbins, the middling quality becomes more and more conspicuous.