I second matt7591's suggestion of renting first.
If you want to drive through big cities you will find the Serenity to be cumbersome in terms of both length and width. Parking will be a problem - even parking garages will have height limits. Some cities will be extremely difficult - think San Francisco and maybe Boston.
Your comparison is rather strange - the Adventurous is built on the short Sprinter chassis. If you are concerned about space you should also consider Sprinters built on the longer chassis.