Owning all three of the fisher coils you mentioned I will add there is not much to gain by a 15 inch coil, except you will cover more ground, The 11" inch coil is my goto coil for most situations. The Small coil is for doing a small trashy area, around a lot of bushes or if I want to hike in somewhere. I also have found extra depth can only be gotten if you set the machine up properly and have good soil conditions. Swing speed changes with the mode your using and I am assuming you have little experience with the F75. I have a routine that produces for me. Do what has worked for you in the past and stick with it. I will be watching your reply on the Neil coils as I have not had a reason to buy one as my Fisher coils do the job. Again try burying a coin on site to get a good feel for how your different modes work and at what depths. Then your first good deep target repeat the test to see if your getting good tone. You will see tone get better or worse depending on the mode and settings. Running sensitivy at 99 which can be done now may produce a coil overload and cancel out a good signal. Not all signals of course but I am not comfortable with the sensitivity set over 80. While FA mode works in iron infested areas, I still like JE mode when I can run it. BP mode is great for checking targets if you getting a iffy signal, but not a mode I hunt in unless Im in a very clean area. That 15 inch coil is just as deadly as the 5 inch coil. But does throw the balance of the machine off, Sure looks impressive though when your buddys hunting with a smaller coil lol. Let throw one more thing at ya. If you dont hunt this spot regularly then your not going to have a base to which to gauge targets and depth. You would need to hunt several times to get a better feel for which coil does best. DE mode is the fastest mode, JE mode is the sensitive, BP mode is the deepest, FA mode requires a slow coil swing.....ect. Target and soil condition will change so there is no one answer to your question. I hope this helps you.
