Here we go.

Bike shorts (or bibs) are tight to you don't get any friction where you don't want it, if you know what I mean and i think you do. They fit like a second skin, as opposed to having something just lightly tear at some tender spot for hours and hours until you think waterboarding sounds like relief.
The padding isn't padding so much as moisture control. If you've done Levi jeans on a really hot day you know how friction and moisture can add up to no fun.

Bib's just stay in place better. There's that "man bra" thing that can be a plus, but mostly it' stht they don't slide at all. I never am aware of the straps. They also can come up your back better when you're all bent over.

BIBS. Get 'em.