If you rent at the parks, chances are very good you won't have to wait for another second Disney bus; because, there are only two slots to fit a wheelchair and/or
ECV on each bus. This can happen and then you wait and wait for that next bus. It can be a little intimidating to park a
scooter on the bus to get it ready for travel especially if you are the second one trying to get it into the designated area for it. You have to do a lot of twisting and turning to get it into place correctly. The driver needs to harness it; so, it has to be correct in it's place. You get the hang of it after a while, but it's hard at first. For my MIL, we did it for her all the time.
If you are planning on enjoying the resort a lot like utilizing the food court, pool, etc. then having a scooter all the time is very nice especially if you don't get a preferred room. Not walking to the bus from your room everyday is nice, too. The downside is you have to charge it everyday. This means for most bringing the scooter into your room each day. It will block part of your room. Make sure you connect everything correctly; so, it's charging correctly. You need to keep track of your gage; so, you know if your battery is getting low. If your diligent and shut off your scooter each time you eat, see a show, parade, fireworks, etc. your charge will probably last for the day, but if you get lazy about it then you could have an issue. You should always have your recharger with you in the parks if you are doing an outside rental. You can in an emergency use a Disney outlet at the park to recharge. if something goes wrong with an outside rental you will wait some to get a replacement. With a Disney rental a CM will assist and you will get another one quickly. With an outside rental you are pretty much stuck if something happens. You can't exactly desert you scooter; you need to wait for the outside company to get it back for another one.
It is definitely more expensive to rent from Disney. There's a company right now charging $150/week for their travel scooters. We used them in Dec. 2009 and we were quite satisfied.
I'm a recent convert to outside rentals. It gave us more freedom to visit other parks like US, IOA, SeaWorld and Busch, but we had a car, too.
For Disney only trips, I still consider it a toss up. I like to walk some; although, all day in the parks is out of the question. Going back and forth to the Disney buses and to the entrance of the parks I can handle. I'm not crazy about the scooter being in my room. Most of us block our door with it and that really is a fire hazard. I like the safety net of getting a Disney replacement scooter if the worse happens (we had one experience this way) and you have a breakdown. Is it worth $48/day to rent a Disney one? We did have to wait once for a Disney scooter; because, we arrived after a 11 am in the park. We got one about two hours later. They will take your cell phone number and call you when one is ready.
There are definitely pros and cons to each way. I think you have to think about how much you need one. If you need one most of the time, then getting an outside rental is probably best.