I own a Super 12, along with the D2, and Eclipse 5.3 and 950 coils. As Magic says the Super 12 certainly has its advantages. You can cover more ground with it. Another application is when you have a thick turf or ivy that makes sweeping close to the ground impractical.
My own observation in the field is that the Super 12 is very sensitive to EMI and ground minerals. The farm field I have been detecting is low to moderate in both of these. When I used the Super 12, I had to lower my RX and both the Discriminate and All Metal sensitivities quite a bit to stop the false signals and keep it stable. Any depth advantage was lost by the lower settings. The only advantage was the larger sweep area. Based on this, the D2 would be a better choice allowing both a larger sweep area and higher sensitivity in these conditions.
I think a better choice for a second coil would be a smaller coil for trashy areas, such as the 4x6 DD or the 5.3 Eclipse.
Just my opinion!