We just had a lot of experience with this one. We didn't rent a stroller the first few days we were there for our 8,6,and 4 yo. However after carrying my 4yo all over Epcot on my shoulder, we had enough.
My 4 and 6 year olds were very comfy riding in a double stroller that costs $15/d. A couple of times my 8yo hopped in and made the 4yo sit down by their legs, but that was cramped.
Your 5yo will be very comfortable in the single stroller.
They had nets on the back of our double stroller where we could stuff all our coats, bags, etc. Plenty of room for stuff.
By the way, you can save some money if you buy the multiday rental.
Have a great time.