Okay. The first thing that I consider is soil makeup. Are the minerals present primarily magnetic, or are they conductive? What sort or GB numbers do you see with Tracking turned ON?
What sort of targets are you looking for most? Are you going after high conductors like silver and copper, or are you looking for low conductors like gold or lead?
What's the trash density like? Is it sparse, or is there a lot to sort through?
How much time can you devote to the sites you're hunting? Can you spend several afternoons there, or is it a one shot deal?
How deep do you want to go? How deep do you think you need to go to get to the targets you're looking for?
These are all questions I go over in my own deliberation about what coil to select.
Of the two coils you've mentioned, I would most likely choose the 10.5" 7.5kHz DD. IMO, everyone should have one in their arsenal. The way that the Xterra is designed and programmed the MF coils in general are the most honest and the best "all around" frequency for general detecting. The 10.5" DD tends to typically run pretty close to the same depth as the 9" CC, but with the added advantage of better separation, and better performance where higher mineralization is a factor. It's biggest drawback is it's weight (a bit heavier than the 9"), and it's not being waterproof (should you desire submerging it).
One other thing to note about the 10.5" coils, is that they don't balance on the end of the shaft like the others. They have a point of attachment that is behind center, making them effectively nose heavy. Some people aren't bothered by it, but if you're not one of those people there is a way to mitigate that issue. Simply mount the coil backward. It will function the same way, but move the weight back closer toward you taking the leverage off the front.
I hope that this helps. If you have any other questions, please feel free to ask.
GL & HH!