My answer is neither.
If you rent a full-size, four-door sedan (Buick Century or Dodge Intrepid or larger), you can break the rental
ECV down into five pieces, the heaviest being 39 pounds. It will fit in the trunk. This type of car can carry six adults and it is much easier to get into than a van for a person with mobility problems.
Although Disney Transport does work and can USUALLY take the ECV, having a car available tends to be much more convenient. This is especially true if you are staying at a park until closing, or going at a time when the buses are crowded.
Also, Disney Transport is most effective if you are going directly between a Resort and either a Park or Downtown Disney, or betwween Parks. If you are going from one resort to another, such as for dining, then having your own transportation is almost mandatory. Can you imagine waiting for one bus, getting to a transfer point, and then waiting for another bus with the ECV. If either bus does not have a working lift, or is full, you will have a long wait; what if both have problems?
Also, from Boardwalk you can easily take the "Friendships" to either the Studio or Epcot (or use the walkways); you would only need tansportation for Animal or Magic Kingdoms or Downtown Disney, or of course to get to almost any other Resort.
The car would be more convenient (and cost less) than a van.