Dave,
I have both the F-75 and the F-70 My F-70 has the same 11" coil as the F-75 I have done quite a bit of testing comparing the two machine. The F-70 with exact same coil that is on the F-75 makes the machines nearly equal in performance. The three things I like about the F-75 are the trigger, the backlight, and the on-the-fly depth meter. The F75 has DE process, so does the F-70, the F-75 has JE process, the F-70 has SL mode. Unless you are in ideal conditions both machines are difficult to use in these modes. Yes, you can dial down sensitivity and makes some adjustments to both machines when using these modes but I find myself sticking to DE mode which both machines have. The F-75 has PF mode which is a favorite of many users; however, I have not found much difference between DE & PF. With the F-75 you can manually ground balance the machine which comes in handy when you are trying to "fine" tune the machine, with the F-70 you must use the "ground grab" feature which does a very good job by the way. The F-75 has a "static" all metal mode which lends itself more to the relic hunters, the F-70 has a motion all-metal mode. The nice thing about the all-metal modes of both machines is that the VDI display works in all-metal mode. I cannot comment about the F5 but I understand it is also a very fine metal detector.
If you are primarily and inland coin hunter I think the F-70 will give you the same performance as the F-75.
If you are a coin and relic hunter than you may want to consider the F-75 because it has a few extra features that appeal to relic hunters.
If you are a beach hunter you may want to go with the F5 based on what I have read.
Comments are always welcome,
Leo234