Neither guy is that big of a difference maker.
Bibby is more likely to hurt Miami than help him due to his abysmal defense and the likelihood of stealing minutes away from a more capable player in Mario Chalmers.
Troy Murphy is a nice offensive player and solid rebounder but he too is an awful defensive player which limits his value. Also, the vast majority of his rebounds are defensive boards -- below average offensive rebounder -- several of which (def boards) he takes away from his teammates. He's not as impactful a rebounder as his numbers suggest at first glance.
Troy Murphy is one of those guys who's stat line is a lot more impressive than his actual impact on his team.
That is not to say Murphy won't help Miami, he will, but he won't be that large of a difference maker. He is fairly equal, in overall impact, to a Big Z or Dampier, just with a different skill-set. He'll supply some extra depth but he won't provide a drastic upgrade.