7500-8000 pounds loaded TT, bunk house for a family. Skip both and get one with the Max Trailer Towing package. You'll want the extra 350-450 pounds of payload for a family, some camping gear in the bed, and 900-1000 pounds of tongue weight.
That package includes 3.73 axle ratio, and is not likely to be found on every dealer lot, or one within 200 miles.
As for 3.31 vs. 3.55: I favor the shortest (numerically highest) axle ratio possible with gas trucks. The real world fuel economy difference is not measurable. The extra wheel torque is always butt dyno approved.