Ask yourself a few questions:
- What hazards will you encounter?
- What gloves will you wear?
- Where will you be able to reach a knife most easily?
- Will you spearfish, or will you look at fish?
- Do you enjoy gear maintenance?
- Would you endanger yourself to save your knife?
Hazards: I see you're from Idaho; will you be diving locally in lakes/reservoirs? If so, your main cutting need will probably (not exclusively, but probably) be dealing with monofilament fishing line. A line cutter (e.g., Trilobite) will do nicely for that. If you're going somewhere that might require you to cut something more sturdy, or to use a knife as a prybar, then get a fixed blade.
Gloves: You wrote that you're currently wearing 5mm gloves. Those are inherently clumsy, so manipulating a folding knife can be challenging. If you're going to continue diving cold waters, I'd go with a fixed-blade knife, preferably one that locks/straps into its sheath and that's deployable with one hand. If you're going to travel to warm waters, though, you'll probably go bare-handed or just wear thin gloves, so a folding knife would be easy to deploy.
Reach: Whatever cutter you get, put it somewhere that it's comfortable and convenient
for you. A BFK will fit nicely on the inside of your calf, preferably on the same side as your dominant hand. It's less likely to catch on entangling hazards there, and you just have to draw your leg up/reach your arm down to find it. A small knife should attach anywhere that requires a short, non-contorted reach; a BCD's corrugated hose is one popular location. If you have a wrist-mounted computer, get a line cutter and put it on the computer's wristband.
Spearfishing: If you're going to poke fish with pointy sticks, you want a straight-up dagger to finish them off. Otherwise, go for a blunt-tip knife. You're less likely to stab yourself (or your BCD, or your drysuit) while putting it away.
Maintenance: If you're the sort who loves to love his gear, get a stainless blade. If you want to rinse-and-forget, get titanium, or just get an inexpensive stainless knife and rinse-and-forget until it gets rusty.
Endangering yourself: Remember that a knife is disposable equipment. You want one that you won't cry over losing, or that you'll do something risky to retrieve once (not if) you drop it. Save your money for something really important and that won't tempt you to endanger yourself.