Definitely go with the professional route and the insurance. You wouldn't want to learn jiu-jitsu from a guy who saw a UFC once, and you don't want a legal document to protect you and your assets from a guy who read and signed one once. I'm sure there are a few lawyers on the board, but we are here because we are martial artists in some form, not legal experts.
As a general rule though:
A waiver keeps honest people honest. It acts as a deterrent for lawsuits. (I did sign a waiver, I guess it is my fault I broke my wrist . . . ). However, as mentioned earlier, anyone can sue for anything. The waiver will provide some coverage in court, but there is no waiver for negligence or stupidity. (Not saying you would be either, but just making a general statement!) Liability insurance helps protect you and when the waiver fails and you are held financially responsible for injuries or such, you have a backup.
Also, the one thing not mentioned is that I would definitely seek out a business lawyer to help protect your company. Sole Propietorships, Corporations, LLC, LPs, etc. all offer different levels of protection to you as an individual and to the company if someone does wish to sue. Again, if you are completely negligent, you can't hide behind a corporation, but accidents do happen and the correct business model can help protect you as an individual as well.
Failing that . . . stick to the old wolfpack method...
Someone goes down with an injury, kill them and hide the body. There will be no one to claim you owe them a few thousand dollars to cover the cost of the broken bone. It's a tough business model, but does weed out the weak . . .
