Arguments for Driver's Side:
Less of a blind spot
Swing away from curb during loading
Arguments for Passenger Side:
Better balance of truck due to fuel tank and driver
Swing away from traffic
Factory License Plate Bracket fits
Arguments for centering:
It's all a blind spot
Weight is centered
Must place license plate in center of tire like a Jeep
Can swing left or right or even down
Arguments for on the roof:
Center of gravity is higher for easier servicing the undercarriage in corners
Roof is rated for 150 LB
UV light breaks down the rubber so you can get new tires sooner
Arguments for underneath:
It acts as a smooth slide when going over rocks
It allows as a protection in rear end collisions with Honda Accords to protect the rear differential
It's easier for short people to change a tire because it's closer to the ground
You don't have to spend extra money for a new bracket or bumper unless your tire is larger than a 33"
Arguments for interior:
It protects the tire from harmful UV
You get the lovely smell of new tires every day
You get to see how your tire is doing every time you look in the mirror
No one is going to steal a tire if they have to break a window first
It's easier to get out for a change because gravity helps get it to you