Parking, getting to and from the parks, etc. is more when you stay off site.
Really? I disagree. We went to Disney twice in 2009. Stayed at POR one time, and rented a large home offsite the next. My time spent getting to and from the park (comparing when I walked out the gates in the evening and arrived at my accommodations, and reverse in the morning) was FAR less staying off site. I NEVER had to wait for a bus (which I always did using Disney busses...sometimes more, sometimes less, but always at least 5 minutes, average was more like 10-15, not including the walk to and from my room to the bus stop, add another 5 minutes to that). When I stayed off site (the Emerald place...whatever it's called..LOL), I was IN the park at AK in less than 15 minutes from getting into my car in the morning, and returning home. Epcot and DHS, about 20 minutes each, and MK was about 30 (by far the longest because of having to the TTC thing). And, in the evening, no wait to get in my car, drive home, and walk the 15 steps into the door. Believe me, after a long day at Disney, the last thing I wanted to do was stand around a bus stop, stand on the bus for the ride (often there are not seats) and then take that trek back to the room. And, I'm not an out of shape person in the least. It's just tiring.
Then, if you want to go sit by the pool at your resort, it's another trek. At our house, it was right there...a mere 10 steps from the house to in the pool. It was heaven.
Also, for us at least going back and forth in the middle of the day by car (to take a break) was a whole heck of lot easier than the time spent on busses in the middle of the day.
Bottom line, if you stay close to Disney, there is no difference (or even a difference IN FAVOR of off site) in travel time. This has been shown time and time again...witness the Unofficial Guide travel times.
Now, if you stay at a monorail resort, you will get to Epcot and MK faster on site, no question. However, you will have a far longer trip to DHS and AK than off site. And, of course, you will pay dearly for the privilege of staying at a monorail resort.