I'm not one of them. I have been in and out of MD'ing since 1968. My first detector was a home made BF. In the 90's it was a White's 5000-d. In 2006 it was an Ace 250. I have never had money to go out and buy a bunch of detectors. I really got into it with the Ace. Two years with little income and no work. Man, did I have some long hours and great times detecting. Last August I bought a F-70 because I am working and making lots of money. Bad thing is I have VERY LITTLE TIME FOR MD'ING!
That is my Bio.
Now that you know where I am coming from, I will answer your question. I don't want to mislead you and make you think I am one of those experts.
I know my Ace extremely well. I don't leave home without it. We are so close My wife is jealous. We are one and I know all it's little idiosyncrasies.The F-70 and I are past the honeymoon and are learning to work out our problems. We can swing all day and have no problem. Well, we went swinging for 4 hours once. I really don't slow down. I learned that I need to slow down in heavy trash and I do much better with the F-70. We don't communicate well yet. There are too many beeps and blips for my mind to assimilate. I have to go slow or I will miss something. Lots of times I feel like I am missing something. I have done better with my Ace, but I have MD'ed more with my Ace and that loud bong lets you know there is something in the high range even with iron wrapped around it.
I mostly use the small coil and I love it's depth. I'm sure there is a lot of masking with all the iron. I get many targets reading around 10"to 16". When I lay the coil on the ground it is usually 7" to 12". My ground is around 80 with 1 to 2 bar dirt. Cinders is my nemesis. They give me those deep, vague coin hits around silver. Also it love little tiny chips of foil and gives out a strong response. Nothing scientific about it, from the top of my head, I have gotten dimes up to 9".
I think the F-70 is stable. It is just so sensitive. I have had little trouble with emi. I have never not been able to MD an area yet.
I love the stock coil in an open field relic hunting. I can really crank it up with no interference. I also don't get the chatter from all the trash targets. I think it is funny how it can pick up cow patties, but it can't pick up deer pellets.
The F-70 sucks in a tot lot. It is just too powerful. The Ace can get with in 1" of equipment, but doesn't have anywhere near the depth of the F-70. The F-70 seems to go wacky about every third hunt. I have to reset it, than no more problems.
Again my biggest problem with the F-70 is learning what it is telling me. Not stability. Not depth. Not separation. Not weight. The batteries last for ever.