In either case, you need to be an Emerald Club member.
Wtih Emerald Isle, you are getting an intermediate car with the CHANCE of getting someithing bigger for the same price. It is a specific isle in the parking deck. At MCO, you go directly to the parking deck and the emerald isle row to select your vehicle.
With Emerald Reserve, you are getting exactly what you wanted when you made the reservation. (e.g. full size, SUV, etc.). Each has a specific isle in the parking deck. (e.g. fullsize row, premium row, SUV row). You go to that particular row to get your vehicle.
Ed