Since beaches are generally lots of acreage & the soft sand makes things go deep in a hurry, I'd guess what is wanted may be the biggest coil (for fast coverage) that improves depth on single coins & whatever type of jewelry you hope to find.
There are about $3,000 worth of coils for the DFX/MXT/M6 between what Whites & other mfrs make. Most of the non-Whites coils are Detech, sold exclusively in USA by Kellyco. Visiting the Jimmy Sierra, Kellyco & White's sites will give you a pretty good picture of the choices.
I had a Jimmy Sierra 12.5" concentric "Hot Shot" coil on a M6 & it did better at IDing deeper, getting coins on edge & small jewelry. Of course other large coils can probably do this. I would hate to decide for you which coil might be best, as I haven't tested them all. I'd say buy used if you can, then if you don't like the coil as much as hoped, you can sell it without much loss. Most of the used coils are seen on eBay. HH, George (MN)
