Here's a travel time calculator that gives various travel time options. It seemed fairly accurate to me, maybe a bit longer estimated:
http://www.ourlaughingplace.com/aspx...x#.UafpgUBwo9U
We did not seriously consider renting a car. Our reasons:
1. We are public transit people, we'll generally take PT instead of drive whenever possible. Lots of reasons for this, but for us its just the right thing to do when we can.
2. The rare opportunity to not bring carseats. We took the Magical Express to/from the airport which doesn't allow carseats. Shuttle buses don't allow carseats. This was our first time flying without carseats, we did borrow a CARES harness for DD. It was nice not to have them. I'd never take a taxi or rent a car without them, so we'd have that to deal with.
3. We stayed at a monorail resort and were able to take the monorail and not fold the stroller for MK and Epcot days.
4. We would have had to pay for valet parking at the resort as well as car rental, or park super far away and walk even further each day. Just not worth it to me.
We only needed to take the bus on 3 of 7 days - to AK, to HS, and to downtown Disney. The rides were fine for us, never had a very long wait or drive, I was only annoyed because we had to fold the stroller. (would have applied to driving too) The only time the ride was longer than it should have been was because I chose to go from AK to Boardwalk then walk to Epcot because I wanted to see the Boardwalk area. There were several stops and we were last. Had we taken the Epcot direct bus it would have been much shorter.
Again this is one of those issues that will be different for every family. Just sharing the reasons behind our decision in case it helps.