Hi Leandra,
Welcome to the Disney Parks Moms Panel! I am a huge advocate of using a stroller in the parks. My advice to you would be to know your child. With an age difference like that of your girls, I can definitely understand your concerns.
If you think your daughter would benefit from the use of a stroller you have a couple options. You could simply bring your own or rent a stroller from an outside vendor like Kingdom Strollers to have with you all day in the parks, or you could play it by ear and rent a stroller when you need it.
Stroller rental is available in all of the Walt Disney World theme parks as well as Downtown Disney, but you may not remove strollers from the parks. If you plan to visit more than one park in a day, you will need to leave your original stroller as you exit the first park and then present your receipt at the stroller rental location in the second park to pick up a replacement.
My family's preference is to rent from Kingdom Strollers. They will drop off and pick up the stroller from your hotel and I have found that they offer several options with higher-than-usual height and weight limits, which is great for this mom of tall boys! I also confess that I enjoy having a spot to put my bottled water and snacks when I don't want to carry a bag.
Ultimately, there is no right or wrong answer to this question. It really boils down to what will make your vacation more comfortable for everyone in your family.
Please feel free to ask us any other questions you have as you plan. Wishing you many magical memories with your daughters!
Andrea U.