I am thinking of getting a metal detector to look for coins and jewelry.I would like to stay in the $600 price range but would go up some if there is a real advantage to it.What would an ar15 user get?

You might not want to start off at $600 for a machine, you might want to try a cheaper model to see if your going to really get into it. I started off with a Bounty Hunter quick draw, it was like $150 when I got it, and certainly earned back it's share of just pocket change.
All the bells and whistles on $600 detectors you may find you can't use. For just coins and jewelry at parks and beaches you don't need more than a detector with discrimination and depth control. Expensive isn't always going to be better, try some of the Whites, or tesoro's detectors.