Good question, correct me if I'm wrong but I think one thing that separates the good from the great in Pen Testing / Ethical Hacking is that Pen Tester who's willing to go that extra mile. What I mean by this is that Pen Tester that's involved in a Pen Test and actually goes to that extra step and possibly develops an exploit that a possible attacker could develop and launch it to penetrate a machine. An ethical hacker who holds a CEH certification on the other hand might just run through and perform your usual Pen Test. Not saying C|EH's don't know how to develop their own exploits, I'm just randomly saying based on another user who would have knowledge of a person who holds the CEPT Certification, etc. It's that person's experience in the field that I think makes the difference. I don't exactly think there's too big of a large difference between the two (Ethical Hacking / Pen Testing), I'd say it all depends on how qualified the guy / girl is your hiring and how into it they are.
eCPPT (Silver/Gold), GSEC, GCIH, OSCP, OSWP