Like Bruce mentions it all depends on how you use your space. Remember that you'll probably be camping long after the teenager leaves the nest so personally I would plan on going smaller say 26-28 ft. Teens can manage easier than adults so they're flexible.
In the 2500 Sunseeker I would also go with the optional sofa with removable table. It's more versatile than the banquette. You can put your feet up with the sofa or turn it into an extra bed. The banquette doesn't offer those options and at best becomes a very short bed for little kids.
