Great advise above, floor plan is definitely the start point.
Following that comes lots of research on each manufacturer, their construction methods, thought into space management, quality of build, and their emphasis on customer relations. Some manufacturers just stand out from the crowd.. Don't be blinded by the "ohhh it's new and fancy" and forget to really get into the bones of the unit, as it's extremely easy to do (we may have made that mistake). New does not mean better or in some cases even acceptable.