I had the same question and started a thread on it here:
Poll: Dive Knife vs. Diving Shears???
Here is my summary from that thread of the discussion. Feel free to comment/debate as you see fit.
Diving Shears
Pros-
1. Easier for cutting monofilament line
2. Less risk of injuring yourself in the process of using it
3. Easy to cut one handed
Cons-
1. Harder to manipulate with gloves on (can be dropped when grip relaxed)
2. Not ideal for cutting thick line such as rope
Dive knife
Pros-
1. Serrated edge can be used to saw through thicker material
2. Easy to use and hang on to with gloves (maintain positive grasp during use)
3. Spade tip allows for prying, if necessary
4. Easily cuts thin line, if line is held tight
Cons-
1. Not always easy to cut one handed (usually one hand needed to hold the material to be cut)
2. Higher risk of cutting yourself
3. Not allowed at some dive sites (to keep divers from prying open mussels, for instance)
"Z Knife"
Pros-
1. Great (possibly best?) for cutting thin line and webbing
2. Can be used rather easily with gloves on
3. Lightweight
4. Small
Cons-
1. Need two hands to cut unless the material is taut
2. Razor blade can corrode and dull faster than normal knife if not properly maintained (but is easily replaceable)
Fogcutter
Pros-
1. The best of both worlds: a knife and a pair of shears
Cons-
1. You wind up with a decent knife and a bad pair of shears
2. Difficult to manipulate with gloves on
3. Two tools, yes, but no redundancy