I live in East Tennessee and have the red clay and mineralized ground.
I have owned the DFX, SE, and F75.
The DFX is a great detector and I found many nice things with it. I bought one as soon as it came out. It is very well built, well balanced and is a real work horse in the field.
I have had an Explorer almost as long as the DFX. First I had the XS and now the SE. The Explorers have been the most popular detector around during that time and for good reason. The are great detectors that have great depth and work well in bad ground. The also do very well in iron infested ground. To some they are a little heavy, but for me that was over hyped and I can swing one most of the day without any problems.
The F75 is the new kid on the block, and so far I'm very impressed with it. For the first time since I first got my Explorer(s), it is a detector that I reach for first instead of the Minelab. It has a very fast processor, and can separate targets better than anything I've ever used. It has great depth and works very well in iron. It is very light and well balanced and just plain fun to use. The TID is very accurate even in all metal mode, which is another thing I like about it.
I still will not say that the F75 is better than the SE or vice versa. I will say that the two of them compliment each other very well and I plan to keep and use both of them. Where the SE might be a little slow to use, the F75 is fast. Where the F75 might not handle the mineralized ground quite as well the SE can work it's way through it.
I've sold my DFX. Even though it is a great detector in it's own right, I just wasn't using it as much as the other detectors I own. I look for White's to release a new top of the line detector possibly this summer or by Christmas. I will have no problem buying another Whites machine and look forward to what they come out with.
For now, I like the F75 and SE. They are two different but effective tools. If you are struggling to make a decision I would buy a F75 or SE and not look back. If you enjoy it, you will probably have either both of them one day or something else that will be released by then.
But if you get a F75 or SE and learn how to use it, you will find many nice things. Just keep swinging the coil and good things will happen.
Good luck on whatever you decide!
HH Alton