As far as what I know the 5X10 DD coil is lot easier to use where you have "hot rocks" basalt. You can usually ground balance them out with a DD coil to some extent. The "concentric" coil is a tad more likely to sound off on basalt. And the 5X10 DD coil being the HIGH freq coil (18.75khz), it will pick up the smaller nuggets better. So if your looking for nuggets, the 5X10 DD coil is the best for small gold. This "IS" the forum for gold searching, but check out this link for TONS of info on the X-TERRA 705. http://www.findmall.com/list.php?55 Be sure to read the "FAQFAQ" part in the top of the forum. Lots of necessary info on understanding the 705 I hope posting the link is OK with Rob!!!