When I am looking for small gold I don't use the bigger coils, I use the 6" coils for both my gold bug 2 and the Xterra. The smallest stuff that I have found with the Gold bug took 3 small nuggets to show on my scale at .1 gram, and the smallest nugget that I have found on the Xterra has been .2 grams. I might be able to get pretty close to the gold bug with a lot more practice with the Xterra, but I think that I can here the sound of the gold bugs zip or click on the real small stuff easier to here than the rise in pitch of the Xterra.
I have not tried to find micro gold patches, but i would think that if the gold was fairly shallow I would try to use the smaller coils, they don't have the depth of the bigger coils, but they have more sensitivity to the small stuff that the bigger coils won't see. I think that either the Xterra or the Gold Bug 2 would be good choices with the smaller coils and using the 6"DDHF coil for the Xterra 70.
Bill