My previous truck was a 2004 Chevy 2500HD crew cab with 6.5' bed. I hauled a bunch of stuff in the bed, towed a travel trailer, and it did everything I needed. I once had a 20 sheets of 4'X8' 1/2" OSB slide out the back as a I left a stop light. I thought the weight of the boards would grip each other until I heard a huge crash and saw them sliding out the back.
My new truck is a Ram 2500 crew cab with 8' bed. It also does everything I need and then some.
6.5' bed pros
- shorter turning radius
- fairly easy to park
- lot's of accessories availble
6.5' bed cons
- everything must be tied down.
- can make longer loads difficult if excessive weight is on the tail gate
8' bed pros
- can fit long boards inside the tail gate
- no need for sliding hitch on 5th wheels (not an issue for the OP)
- can easily fit golf cart with tail gate closed for pulling TT
8' bed cons
- LAND YACHT. These things take a wide berth to turn and park.
- Much less common than short bed.
Honestly both bed lengths will fit the OP's needs just fine. Oh, and the ability to fit a board 8' long is not the be all, end all. If the OP is wanting to haul long stuff and tow a TT then an 8' bed is probably the better choice. If not, the 6.5' is perfectly fine.