I've got the 11 inch DD and 10 inch concentric. The only place I have found where I have a preference for the 10 inch concentric over the 11 inch DD is over wet salt beach areas. I find the 10 inch concentric a little more forgiving to ground balance to the wet salt than the 11 in DD. In that environment the 11 inch DD wants me to maintain a pretty exact ground balance matching to the wet salt or it tends to give more falsing than the 10 inch concentric. I can hunt successfully with less adjustment with the 10 inch concentric in that specific environment. But I must say, after doing a bit of beach hunting with it, I am now not taking it to the beach as I am spending more time in the water and not so much time where I won't risk dunking a detector.
Most open dirt areas I hunt with the 11 inch DD. I like the 6.5 inch elliptical concentric in trashy areas or up near poles and fences.
Cheers,
tvr