Body lift pros: Cheap, no steering problems, don't need alignment, keeps CG low, allows for bigger tires which result in more frame and axle clearance, makes working on truck easier, allows for use of doubler or fitment of bigger tranny.
Body lift cons: Can be unsightly, lift itself provides no frame clearance increase, flex does not improve.
Suspension lift pros: Better flex, more frame clearance.
Suspension lift cons: expensive, requires alignment, can result in steering problems, raises CG substantially
I have always said, use a suspension lift to get the flex and frame clearance you desire. If you still need room for bigger tires, go with a body lift from there. If you only want bigger tires, a body lift is the best option.