I am primarily a coin and jewelry hunter at turf sites in Ventura County. I occasionally, 2 or 3 times per year, hunt a local beach. I am not an expert at saltwater beach hunting. However, my experience using an MXT / MXT Pro is that I do better with concentric coils than DD coils both inland and at the beach.
A couple of years ago I started a black sand beach hunt with a 10 DD mounted on my MXT Pro. It did fine in the dry sand. When I got to the wet sand I switched to Salt mode, did a complete GB, started hunting and immediately got lots of falsing. In fact I did Ground Balancing multiple times sometimes locking afterwards, reduced Gain but the falsing continued until I moved back to the dry sand. I worked my way back to the car, mounted a 950 coil and returned to the wet sand. I did the same GB procedure with the Gain still fairly high and the falsing happened so seldom that I can almost say it ended. Yes, I found some good targets in the wet sand with the 950.
I have never tried the 6X10 at the beach however it works really well on my V3i for turf hunting. By the way, the 10 DD from above works great on the V3i in turf. However, the Eclipse 950 also works better at the beach on the V3i than the 10 DD.
For truly expert advice about coil selection in your hunt sites I suggest you try to find a detecting club. I know you are locate somewhere on the central coast so hopefully one of the clubs listed below is near you.
Treasure Hunters Society of Santa Clara Valley
San Jose / Santa Clara, Ca
http://www.members.tripod.com/~waipahuhaole1/treasurehunters.html
Central Coast Treasure Hunters Association
San Luis Obispo, CA
http://www.cctha.org/