In regard to the "paratrooper gravity knife", gravity knives are just as illegal as Automatic knives in most area's.
I have owned about 4 auto knives in the past year. And I did carry one of them quite a lot, and did until I lost a part when i took it apart to clean it. I knew what I was doing was illegal, but as it wasnt my main defensive blade, it wasnt even a defnsive blade, it was a risk I was willing to take.
However I think that for your situation, the one hand opening folder would be a better choice.
My main carry is an Emerson Commander. It has a thing on it called the Wave that pulls the blade open as you pull the knife out of your pocket. No springs, and it doesnt use gravity to open it, it uses the force of the action of pulling it fromy our pocket. So in effect its open before you can even press the button on an auto.
Also if you are a CCW holder, and you get caught with an auto, buh bye CCW license in all likely-hood.
I say go with a one hand opener and leave it at that.
I reccomend the Emerson Commander, but at $220 factory retail and lowest online price I have seen being $165,(unless you want to shop at Botach tactical, who has it for around $130 or so I have been hearing) its out of some people's price range.
the two mentioned, Spyderco and Benchmade make wonderful knives as well.
Also take a look at Columbia River Knife and Tool, they make a heck of a product for much less than Spyderco or Benchmade.