I carry both a dive knife and shears. Especially with where I dive now. I've discovered that there are delicate situations where a knife just isn't "welcome", like when working near gear or getting out of fishing line in the lake. I seem to get less frustrated if I manage to get tangled in something when I know my shears are on me.
Shears: can get you out of a tight jam of fishing line, net, etc. that a knife may not be able to get into and/or may not cut through. One hand operation in most cases. Less need to look where you snip once you're on your target. Least likely to hurt you or another diver if dropped.
Knife: great tool if used properly, but relies on the force of the human body for cutting ability. The sharpest knife may not cut if enough force is not applied. One hand operation in some cases, but primarily requires a second hand to steady a target. Should maintain visual contact with the target to be cut as a knife may slip. Can hurt you or another diver if dropped. Great UW defensive tool if needed in a "last resort" situation, though I would not recommend that. Can be fashioned into a make-shift spear if needed for spear fishing (not w/ speargun for you rednecks). Flat tips can be used as a screwdriver or prying tool.