Bed length is a personal choice.
I've had several of both over the years.
One thing I've noticed non of the longer wheel base trucks I've had rode/handled any better than one with a shorter 140" wheel base trucks.
Now years ago when a short bed truck had a 118"-122" wheel base and a long bed truck had a 130"-134" wheel base then sure the longer wb truck handled better.
With todays new 5th wheels with the notched rounded front corners a slider isn't needed
Also some GM trucks have a longer dimension from the back of the cab to the trucks rear axle which eliminates the need for a sliding hitch except some of the older square corner flat profile trailers (like mine).
Get which ever bed length/wheel base you like. One isn't superior over the other for every one.
You won't have any handling issues with todays 140"+ short bed trucks.
Bed conversions ??
Most of the time those types are for a dedicated tow truck. If your like me and have other trailer types and need a pickup bed to carry things they won't work.
But they sure do look good.