From my experiences so far with the F-70, I think the standard concentric is the best full size coil for the F-70. It seems to be better with depth than the DD coil but I will say that the DD coil is very good in light to medium trashy areas. Not as good as the small 6 inch coil but for covering large expanses of trashy areas, the DD coil is just a tad bit better (in my opinion) than the standard coil. But for overall depth, the 10 inch concentric (the standard coil for the F-70) is hard to beat.
My deepest find so far with the F-70 is 13 inches with the standard coil...it was a penny that was highly corroded and had a green halo around it in the dirt with lots of trash around it. It was a copper but I could not read the date or see too much detail because it was so corroded.
I have the Standard coil, the DD f-75 coil and the small 6 inch coil.
Here are my preferences on the coils:
The standard 10 inch concentric coil for overall depth. In some instances almost as good as the F-75's DD coil for separation in trashy areas. The standard coil can be erratic in trash. So far my tests have shown that the standard coil is one and a half inches deeper than the F-75's DD coil on the F-70.
The small 6 inch coil for really trashy areas. I am beginning to love this coil. It seems to be able to pick out the good targets from the bad very easily. I have found that the 6 inch coil gives up just a little on depth but what it loses in depth it makes up for by being able to find the good stuff.
The F-75 DD coil...for relic hunting and to cover large areas of ground. Great on separation and very good on depth. In my opinion, not as deep as the standard coil but close enough. I like using the DD coil in light to medium trash areas. For heavy trash, the 6 inch all the way.
I have not had the chance to use the SEF coil on the F-70 yet. I am thinking of getting one because it is so big and would really cover a lot of ground. I have a Spectrum with a Bigfoot coil but I prefer to use the F-70 and the SEF coil looks to be a real coin sweeper and from what I hear, it does not give up depth like the Bigfoot coil does.