with that many people - I would definitely look in staying onsite at WDW. Pop Century is nice and fairly reasonable. Yes it will be more expensive. but not when you compare it to hiring 5 taxi to bring everyone home because you missed the last bus. or worst your group won't fit in the last bus.
don't stay offsite unless you are willing and able to drive.
there are some nice bargins - but offsite is generally better if you have your own transportation.