A couple of thoughts.
Do your storage options change based on the size of the vehicle? Maybe free at home for shorter trailer?
If you're the type who constantly swaps out vehicles I would go with the shorter one as you'll probably swap it out for a larger one down the line.
If you hold on to your vehicles until they are literally worn out, then get the larger one because as the kids grow they might want more room or maybe want to bring a friend.
Sit in the bathroom and close the door and pretend to wipe. Is there enough room? I'm serious on this one, it will matter down the road after the newness wears off.