I have a couple of Duratote and Fortiflex boxes and love them, they do hold a lot. I also have a couple of totes that I like. The Duratote and Fortiflex boxes hold more stuff and are easier to clean out than the tote bags. The tote bags are easier to carry if you have your hands full, you can just hang them over your arm, and you can also hang up off the ground on hooks. I like both styles!
The Duratote with the step stool cover is a great idea, especially if you have tall horses, like I do, also, the step stool cover keeps the stuff in the tote clean when you aren't using it.