You're talking a difference of about 1100 lbs in weight and 4 ft in length. You don't say what you're going to pull them with, but in either case, make sure your tow vehicle is not overloaded.
Longer units are a little bit harder to get into some places. But since most people seem to move "up," you might be better getting the 2511S. Bigger bathroom, pantry, more kitchen space.
For the 2511S bath, they do make foldout shower curtain rods that give you a little bit more room. It flips in when not using it and flips out to give you more room when showering.
In the end, only the two of you can decide what works best for you.