For most of my dive gear, I believe in having equipment that is reliable and easy to care for. However, in the case of a dive tool (a.k.a. knife), I don't see the need to buy something very expensive. I have a stainless steel blade from UK that has served me well for about 6 years. While it does have a little rust, it will still get the job done. As far as I remember, I have only ever had to take it out of the sheath once when an angry salt water croc tried to take a nip out of one of my fins. The other thing about a dive knife is that if you buy a cheaper knife, you won't care as much if you lose it or drop it. If it is an expensive knife, then you are going to be tremendously upset if you drop it. Some folks will go as far as trying to retrieve it when they shouldn't, putting themselves or dive buddy in danger.
In my humble opinion, go with a cheap-o dive tool and get a good set of EMT shears. My shears are invaluable. I have used them many times cutting through fishing line or wire leaders.
Just my two cents.