Congrats on the X-70. And don't worry about the concentric coils. Once you start using them, you will find they pinpoint tighter than the DD. Just be sure you overlap your swaths a bit more.
My mineralization is moderate around here. So I use the concentrics 95% of the time. I don't hunt Civil War Sites as there weren't any battles fought around here. But I do "accidently" dig quite a few buttons, bullets and pieces of brass when I am out hunting for coins. I prefer the 3 kHz because it seems to be "hotter" on the higher conductive targets. But, the 7.5 isn't a slouch either. I don't use the 18.75 too much as I have found I have to lower the sensitivity on the X-Terra or it chatters. Guess that is because the 18.75 is tuned to be a bit more sensitive to begin with. And if I have to lower the sensitivity of the X-Terra to compensate for a more sensitive coil, it just don't make sense to me.
Although the 3 kHz and 7.5 kHz both are hot on spent casings, the 7.5 bounces around a bit more on the TID of brass, compared to the 3 kHz. Like I said, I mostly hunt for coins in old sites. Primarily old fair grounds, picnic grounds, settler grounds and Chautauquas. Some of these old sites have hundreds of spent casings and lead slugs left in the ground. This will probably sound nuts to you, but the bouncing around is what keeps me from digging the brass. Especially old shotgun shell casings. The TID just doesn't lock on as tight on the 7.5 as it does with the 3 kHz. In other words, I "accidently" dig more casings with the 3 kHz than the 7.5 only because I can ID them better with the 7.5. The lead, however, will lock on and read similar to a coin with either coil.
My best advice would be to try them all on the targets you are looking for. If not in the field, at least on the bench or in your yard. Scatter a few around and see what kind of depth and sensitivity you get with each one. And after you decide which one works best, let us know. I'm always interested in learning more about targets I don't typically run across around here. HH Randy
