From what I have been told, there are going to be several coils released for the X-Terra 50. However, they are still in the development stages and no one has told me exactly what will be availble or when. In the X-Terra operator's manual, you will find drawings of 4 coils. Going by the looks of them, I would say that they include the 9-inch concentric, a 9-inch DD, a smaller DD and a larger DD. If that is accurate, those would be 3 that are not currently available. Now, keep in mind that these are not actual photos. Just artist renderings. So the DD / concentric may not be accurate. And, I don't know the exact sizes. But, from what I have been told over the past few months, in addition to the two 9-inch concentric coils currently made, there will be a smaller one and a larger one. Like I said, I don't know what frequencies any of them will be, or which ones are DD and which ones will be concentric. Hopefully Minelab reps read these forums and see our "want list".
I don't actually relic hunt. But, I think the 7.5 kHz coil is what you need, from what you described. It is a deepseeking, lightweight, sensitive coil and handles mineralized ground very well. I have found the higher 18.75 kHz coil to be a bit noisy, even in my moderately mineralized ground. I can compensate for the chatter by lowering the sensitivity. But doing so causes me to lose a bit of depth. I prefer to use the lower freq and hunt DEEP!
Can't comment on the M6 or if Whites will roll it and the MXT into one unit. When you look at the number of posts on the X-Terra, Whties M6 and Fisher Edge, the X-Terra seems to be the most popular, hands down. I have owned a couple MXT's and they are good machines for what they are designed to do. Being a coin shooter, the relic mode and prospecting mode were an "over-kill" for me. I tried out a Fisher Edge this past week, and sold it. I will not go into details about my thoughts. Just that it wouldn't have replaced anything I currently have.
I have several detectors, and as you mentioned, all of them have their place. If I am in the mode for some SLOW hunting in an area that has both old coins and serious trash, I take the Full Band Spectrum Explorer and the stock 8-inch DD coil. If I want to move a bit faster, I use my 6.59 kHz XL Pro and either the 5.3 or the 350 coil. If I am in wide open areas, I like the feel and speed of the 7.5 kHz X-Terras or the 5 kHz Advantage. I have learned the sounds of the Advantage, which makes not having visual ID "no big deal" to me. All of them get used. Depending on the site and what I am looking for.
I don't know what kind of detectors you have, but I have tried about all of them over the past 33 years. If you have some specific questions about any particular make or model, send me an email. I will be happy to share my thoughts about them with you. HH Randy