McAlisters are bigger - I wear 10.5 Clarks and 11 in Mcalister. There's a little wiggle room in the Mcalister toe but the 10.5's were too tight. The Clarks fit me perfect at 10.5.
Quality? Well, J Crew's are made in Italy with smooth leather interior and Clark's interior looks cheaper. I own both so I'm not claiming one is better than the other. I do know that I tend to take care of my McAlisters better and wear them with nicer clothes (they also have a sleeker profile.) With the Clarks I kick them around and don't worry about it because they are cheap & easy to replace.