I have never used one, but I do love that 10X12 DD and that scanning field is wide enough for me and deep when mounted on my Vaquero.
I have read a bunch of posts from guys that do have the 3X18" and they all seem to sing it's praises no matter what Tesoro it is mounted on from the Silver on up.
These would be mostly coin and jewelry hunter types that hunt wide open areas without a ton of trash in parks and sites like that.
Every one of them have mentioned that this coil is no depth monster, however, the 10X12 will go deeper, but I am a jewelry hunter and the wide coil does go plenty deep enough to find the good stuff which in my experience usually doesn't hang out much deeper than about the 6" depth level more often than not.