I would get the city mini car seat adapter and take the city mini. You want to have as many options as you can. You will need the carseat once you are there, so having the adapter makes it easier to navigate through the airport and gives you the option of having both the carseat and the stroller to sleep on while at WDW. The carseat could get very hot depending on the weather, I will try getting him used to nap on the stroller before you leave, it is roomier cooler and has better shade. A baby carrier is a good idea, but depending how heavy your baby is it can be pretty tiring having him nap there if you are not used to baby wearing.
I will also consider bringing the car seat base with you to install on the rental car, it will make your life easier.
I did not go to WDW until DS was older but travelled quite a bit with him when he was little, one of the best tips I got was to ship all baby necessities such as diapers, wipes, etc ahead of time using diapers.com.