We went back and forth on this a LOT for our last trip. No children here, but in the end we decided to rent a car and I, personally, will never go to the World without one again. We traveled in February, the Saturday after President's Day.
We were always able to get on the first tram that arrived, even leaving at park closing. I do think there is sometimes slightly more walking because I felt like the trams were further away than the buses (or maybe I just felt that way after a long day at the parks.)
For us, having the flexibility to just hop in the car and go vs. having to wait and wait and wait on a bus was completely worth it. We chose to take the Magical Express to Disney property and then picked up our rental from the Car Care Center on site. It was great to have the ME so that we didn't have to mess with our baggage and to have that first taste of Disney, because believe me, the Car Care Center is the exact opposite of Disney Magic.
In comparison to the two trips I had taken before relying solely on Disney transportation, we saved a significant amount of time waiting for buses to pick us up and buses to drop us off.