FOr a first trip I woudl alwasy suggest onsite. There is a lot to a WDW vacation, and if your party is not visiting offsite attractions the onsite benefits can add up. Most have been detailed upthread.
For my family it is not just the financial or logistical benefits that we enjoy, it is teh package as a whole. We fly, so this will skew our percepton. WE love the feeling that once we check our bags at the airport, we are on vacation! Yep, no more driving and no more "legs" of our trip.
We are THAT family. We somehow do something one time, and it becomes our tradition, so each trip there might be a "new" bit that gets added. So far, we all have a little dinner out as a family the night before we depart. No cooking, and the fridge is clean. The following morning we are off to the airport bags ready, and DH cringing because he cannot get past the trip when one bag was overweight. His memory is that of all of his unders being transferred to a second bag while all 6'6" of him tried to shrink behind a tiny 4 YO DGD. Finally Arrival in MCO!!! Yay!!! Starbucks and then of to meet the CM's waving their Mickey hands to guide us to DME, and we wave back with our own Mickey hands. DH is again wondering how he managed to be included in the group..... still wondering how his crew who has traveled multiple times here could not manage to find DME on the first try while he is in tow. At this point we have blamed him for our wandering around from one side of that enormous terminal to the other, and he is wondering what fresh hell we will subject him to on the bus.
Our resort is always the last one on the route, no matter which resort we choose, so we get to tour them all. Hey: do not judge that we are easy to entertain, we are on vacation!!! Check in is so easy, no car to park, no bags to drag in, just a visit to the front desk. The first of many, I love DIsney CM's!!! DH is just praying the TV works, and that we do not have a box of essentials tha DD shipped from home. (The one with the matching Disney shirts he will be required to wear. He is still concerned that there may be pictures all over the internets of him in his lime green Mickey head shirt, standing taller then the rest of his crew of shorties, all glowing in lime green. Do not tell him, this year the theme is the 7 dwarfs)
From there is is the best. We use every means of DIsney transportation. No one needs to drive, no one pays for parking or rentals.The magic is from the moment from liftoff to touch down. We are pampered. DH spends the trip wondering how a virtual ticket works, peeking behind walls to get a glimpse of construction, shaking his head at parking lots the no longer needs to navigate, and thanks the rest of us that he can have his glass of wine without impacting his CDL.
So, this is what we pay for when we choose an onsite location.