Atlas is more special, and it would be the one to keep were I forced to choose. The frequency extension and all-around sound is awesome. But no one is forcing me, so I do use the Mammoth from time to time.

Enjoy the mids emphasis on the Mammoth, though I am not implying the Atlas has "sucked out mids", as there are plenty of earphones that would much better fit that classification.
(I think the current Drop price point of $399.00 for the Atlas is a terrific value for those who can purchase through drop.com!)