I got the Crew with a 6.5 just because I thought the 5.5 looks weird, like so much cab and a short bed. I think it looks more balanced.

When I loaded up for a two week trip out in the wilderness, I wish I had an 8 foot bed

 

It's definitely more for the longer bed on a new truck. Used could be all over the place.

 

Agreed too many variables. Trim, options, mileage, year.

The two 2012 fx4's I looked at were the one I bought and a lesser packaged screw with a 5.5 bed. They wanted $35k for mine and $32k for the other. The mileage was within a 1000 kms.