The biggest difference, and this is a pretty big difference, is that AD&D separates race and class. And not only are classes intertwined with race, it means there's nothing like multiclassing. And no rangers (or assassins).
But other than that, I think RC D&D is the best there is. It has a nice skill system, magic item system, it scales up nicer (there are 36 levels so you increase in power more slower per level), there's an optional weapon mastery system that works quite well (much better than AD&D's Weapon Specialization, albeit more complex), the domain management system works, if not fully fleshed out and not perfect.