I think they are pretty much all the same. My brother has MS and has been on a few cruises on various cruise lines. He has had good and bad things to say. My suggestion is to book on the newest ships. The newer ships always have better disabled faciltes.
My suggestion would be HAL. I also concur that whatever the line go with the newest ships and book your HC cabin early if they require one. Also book them a cabin near an elevator and near the parts of the ship they are most likely to frequent most.