In a bounty hunter world you get targets and always keep one item on death if your in a 1+ world.In pvp there are no targets,you can go any where you want in the world of runescape and if you die you lose everything.
The drops are pretty much the same.If you kill your target you get a better drop compared to killing a random person.The higher your ep(earned potential) is the better the drop you get in most cases.