Most will include a rain cover for free as long as you check the "Rain Cover" box when you order the stroller. Most of the authorized stroller rental agencies are pretty similar. Price is comparable, and they all seem to offer a few of the same models. They do each seem to offer one non standard model that the others don't, like Kingdom Strollers offers a BOB option. I'd imagine they are expected to maintain a certain level of cleanliness to maintain that WDW authorized tag. That said, I think each time I've ordered one there has been something not 100% with it. Once the clip only had 2 of the 3 prongs, another time the break occasionally got stuck on the wheel, etc.
I've gotten the insurance, but that's a personal choice. These things take alot of use, and I'd rather not get charged full value for a stroller that just happened to break because of wear. I haven't seen anyone try to challenge the stroller company billing them, but $15 to make sure I don't have to pay $300 is a reasonable fee to me. Especially for something that gets as beat on a stroller.
The stroller may (or may not) be helpful in giving them a place to nap. We usually rent the same model we have at home so our DS is quite used to sleeping in it. Letting the <1 and the 4yo crash in the carrier/stroller respectively could let the 7yo keep going in the park instead of having to make the trek back to the hotel.