Personally- I carried a hefty sized backpack on our last family vaca at Disney. It was not too much, but because of the load, the one day I didn't take it, I didn't miss it. There are some great shots you can get with your camera, but I will be the first to say that everyone needs to take advantage of the PhotoPass. We were able to put all the photos we had with the group on one disk, buy the rights and use as we wanted.
Some people also complain about the bag check lines, but unless it is Christmas, there is nothing to complain about. You have to open all the zippers on whatever you bring in, but still not too much to handle. We took a lot of snacks along and saved a lot of money so we will be doing it again.
I would use whatever you are comfortable carrying. When I switch it up, I sometimes put it down, leave it and don't even notice. Because I am not used to it, I don't miss it.
Have fun on your trip.