The fatboy has larger grates and is heavier. The coal basket is larger too. This means you'd have more room for charcoal.
Yes, they might have same amount of cooking area...they really don't. Ribs fit better on the larger grate. You can squeeze more brisket and/or butts on the racks too.
I have a Party and love it, but then again I'm not so sure I'd try to cook an entire KCBS competition with it. What I think I'd love to have is an extended Party. It's a normal Party that's 3" deeper. That would be super cool.
Life's a party with a Backwoods Party!