Charlie P. (NY) said:
I've got the 6.5" Fisher Elliptical (concentric) coil for my F-75 and it is a real coin hunting tool I seldom use the 11" DD anymore. I find it reaches similar depth!. A dream to pinpoint with and good in trashy spots. No real advantage over your current 5" coil, though.
I am waiting on a NEL 9.5" x 5.5" DD Sharpshooter coil to arrive. I think the slightly smaller DD coil will be a good all-around coil for coins in parks and homesites. Have heard great things on this coil from folks who have them.
If you're a relic or cache hunter a 13" or 15" might be nice.
Another thought is a 1" coil. That's the Sunray FX-1 pinpointer. Attaches to the detector with a little box that lets you switch over and pinpoint - using the detector's batteries, display and headphones. You never forget to bring a pinpointer this way. It's also handy if you want to poke around a foundation or in nooks and crannies in buildings.