The MXT comes with the 950, the MXT 300 comes with the 12" so depending which one you get you'll be "stuck" with one or the other. I think I'd find the 12" a bit too big for my area. Sometimes the 950 is also too big, especially in junk. I use the 6X10 DD most of the time. It's good in mineralized ground & also in trashy areas. It doesn't go quite as deep as the 950 but it's so stable you can run the AC Sens & Preamp gain up higher to help compensate. I also have a 6" round coil that's fabulous in extreme trash.
My choice would be the 12" in non-trashy areas & the 6X10 for normal detecting. If you get the MXT the 950 is a fine coil, too. Definitely the 6X10 DD though.
Any accessory coil you get will find you lots of good things with that fine machine.