Deckert's is definitely cheaper than renting at DLR if you are staying more than one day. I racked up in one month for me and a friend one day and on other trips about $300 for scooter rentals. I bought a used one for only $125 and she is a turbo charged super fast scooter that is well worn but is saving me $2 per trip to the parks and the cost of scooters.
If you do rent at DLR you cannot take the scooter past the security areas but can park hop with the scooter as well as park strollers and wheelchairs.
http://www.disboards.com/showthread.php?p=31683754&highlight=lightyear#post31683754
That is my post of disabled entry points for guests using GAC and mobility devices. At DLR you do not need the GAC unless you have a hidden disability thus guests who use an ECV/scooter do not need a GAC unless they walk onto the ride or show.