The round 10.5" DD HF will definately not find small gold nuggets as good as the 5x10" DD HF. BUT the round 10.5" DD HF will definately go deeper on larger nuggets than the 5x10" DD HF.
Like I said the 5x10" DD HF(sees less ground) will find smaller gold nuggets better and also will handle bad and high mineralization better than the round 10.5" DD HF.
The ONLY advantage of the round 10.5" DD HF is deeper on larger nuggets in bad and high mineralization, but not deeper on small nuggets.
On very low mineralization areas which are very rare in the goldfields the 9" Concentric HF will get the best depth of all the coils on large gold nuggets, BUT it will not handle bad and high mineralization well giving false signals in Prospecting Mode.
DD coils "see" about 70% less ground effect(mineralization) than Concentric coils.
What is the definition of a small or larger nugget in surface area size you may ask? I would say around a minimum of about 1/2"--dime(10c)--penny sized and larger is considered a larger nugget that the round 10.5" DD HF will be have the best depth on of all the coils in bad and high mineralization.
Smaller nuggets than 1/2" are much more common, and there are hundreds more nuggets smaller than 1/2" than bigger than 1/2". Bigger than 1/2" is much more rare. So that is why the 5x10" DD HF is recommended as a first choice coil size for prospecting. Not only for the X-70 and 705, but also for the Minelab Eureka Gold, Whites GMT, Tesoro Lobo Super Trac, Fisher Gold Bug-2, and Garrett Scorpion ALL have 5x10" DD's that are recommend as first choice to use for prospecting as they ALL come with a 5x10" DD coil from the factory.
The round 6" DD HF is not good for general detecting on wide open areas and also is more suseptable to ground noise. Hope this helps.
ross.