Congrats on the IH cent. Silver is always nice to find. But I still get a charge out of digging IH cents. Even after 35 years of detecting!!!
The two coils you have will serve you well for most hunting situations. Concentric coils differ from DD coils in a couple regards. Concentric coils have a detection pattern that some refer to as cone shaped. Theoretically, the pattern represents the signal being transmitted from the outside diameter of the coil (transmit coil) down into the ground and "reflected" back to the inner receive coil. The target doesn't actually reflect the transmit signal. But instead, actually produces one of it's own which is received by the inner winding. Personally, I prefer to think of the detection pattern of the X-Terra concentric coils being shaped like half a sphere. Using your imagination....if you were to take a 9-inch melon, cut it in half and place the flat side against the bottom of the coil, the rounded shape of the half melon would replicate the detection pattern of the coil. A DD coil is two D shaped coils, one a reverse image of the other. These two coils are placed back to back, overlapping one another with the straight portions. The portion that overlaps is the detection portion. Therefore, in a DD coil, the detection pattern is a straight line running down the center of the coil. DD coils use slightly smaller windings than concentric coils and are typically not as sensitive. This causes the DD coils to be less succeptible to interference and are less effected by ground mineralization. So, if you are hunting in areas with moderate or minimal mineralization, concentric coils may provide a greater depth of detection than DD coils. But if you are hunting in areas with higher levels of mineralization, DD coils will help "cancel" the adverse effects of mineralization and provide better depth. Regardless of mineralization levels, DD coils usually provide better target separation due to the more narrow detection pattern.
As to frequencies..... lower frequencies provide better target response on higher conductive targets such as silver and copper. On the other hand, higher frequency coils provide better target response on lower conductive targets such as gold. Lower frequency coils provide more depth on larger targets. And higher frequency coils provide more sensitivity on smaller targets. The X-Terra 70 user has the option of using all three frequencies (3 kHz, 7.5 kHz and 18.75 kHz), as well as 4 different sizes of coils. In addition to the stock 9-inch concentric and 18.75 DD eliptical you mentioned, there are also the 3 kHz round 9-inch concentric, the 18.75 kHz round 9-inch concentric, the 10.5-inch round DD at 7.5 kHz, the 10.5-inch round DD at 18.75 kHz, the 6-inch concentric at 7.5 kHz and the 6-inch DD at 18.75 kHz for a total of 8 coil options. Naturally, larger coils will detect larger objects deeper. And, smaller coils will be be better at target separation. Selecting the best combination of frequency, size and shape is simply a matter of understanding the priniciples of what each type of coil does best.
Each of us has personal preferences based on the type of soil we are hunting, the targets we are hunting for and the ergonomic "balance" of the set-up. Being somewhat crippled up, I find the 10.5-inch DD coils to be "uncomfortable" to use for extended periods of time. I live and hunt in an area with moderate mineralization, so the concentric coils work well for me. And, since I like to hunt for old coins at old sites, I prefer the lower frequencies of coils. Based on the current availability, my choice for wide open spaces is the 9-inch concentric at 3 kHz. I have been using it for well over a year now and find that it is not only very sensitive to the silver and copper coins I find. But it also provides less sensitivity to ferrous targets that are frequently found in the sites I hunt. When I am hunting in the 4-tone, all metal mode, it makes a more distinct "low tone" audio response to iron targets, helping me to make a decision whether to dig, or not. With that said, contrary to my thoughts on the other X-Terra coils, I prefer the new small 6-inch DD at 18.75 kHz over the concentric 7.5 kHz. The DD seems to do a better job of target separation if I am hunting in local parks. In those trashy areas, depth is not the issue. Target separation is. And, the narrow detection pattern of the DD version accomplishes that best for me.
JMHO HH Randy
It's a 1887... 
Not nearly as valueable, as a matter of fact quite common for IH's 
