If you are buying a newer 5th wheel trailer, most come with a tapered front that allows for turning with a shorter bed truck. The longer box will give you more storage but I haven't had a need for it. When buying my 5th, I asked the dealer if I should get a slider hitch and he could have sold me something I really didn't need but he was honest and told me that it wasn't needed. He was right. My truck bed is 6&1/2 feet.
If the 5th wheel trailer is an older one with a flat front then you should probably get a long bed.
Good luck