As a technical dive instructor and knife afficianado here's my opinion:
Scuba divers lose more knives than anything else. They just seem to fall off gear and end up in somebody's elses dive bag or are found at the bottom. Dont invest more than you care to lose.
Diver's rarely, if ever, have an acutal need for an knife. In my 25 years of diving, I can think of once or twice when it was only handy, but never a life saver.
Before anyone jumps in - I've heard that some people have really needed it to exticate themselves from fishing line, and are alive today because they could get to their knife. I've never met such this person though, go figure.
I've seen alot of people cut themselves accidentally on the rocking boats making sandwiches with their dive knifes, buy band-aids and bactine too.
A pair of utility sissors like the paramedics use has been much more helpful than any dive knife. Buy a pair of them, for $15, these are much more handy for fishing line - though not as sexy.
Dive operators are carefull to bring tourists to clean wrecks and reefs where fishing lines have been pruned. On expedition type dives, I carry two knifes and sissors.
Know where you are going to dive and evaluate the risk.
If you must have a recomendation still, I'd say buy a titanium knife. If you manage not to lose it, at least it will not rust or corrode, rendering your beautiful new stainless knife ugly in 2 months.
good luck
TerryL