They both have their advantages and disadvantages. Most people will be more familiar with the option they have and lots of folks have the 6.5' here. I have the 5.5'. Here is what I like about it compared to the 6.5'.
I like that: It fits in my garage (barely, a 6.5' wouldn't), Easier to park, Tighter turning radius (I have a U-turn I routinely make that a 6.5' couldn't make without hitting reverse), Looks sportier (yes IMO). Because its shorter its easier for me to back my boat into its parking spot at my home. It gets so close that I don't think a 1 foot longer truck could maneuver it as needed, I'd have to move the fence to make more room for the truck to swing out.
Now I pull a 28' Omega Yachts Sportfisher offshore fishing boat with mine so it totes the load plenty good enough. I have yet to fall short one foot of bed when hauling stuff but that possibility exists. Sometime in the future I'll have a lightweight utility trailer to serve as a true means to haul larger things as a 6.5' bed is still a short bed.
This is why I chose MY Truck. Don't go buying what works for someone else because it works for them. Make sure you get whats going to fit your needs the best. You will love either one as long as you get the one that fits you best.
) but I haul a lot in the bed and the extra foot defiantly comes in handy.... especially with loads like this
