I had a MXT, but the threshold and weight caused me to look elsewhere. Out comes the F-75 and I bought one about a year ago now. The MXT stayed in the corner all summer, then a gold hunting trip came up. I bought the F-75 because it was supposed to be a good nugget machine, but there were no reviews and I didn't want to get skunked and I was spoiled by the light weight F-75 so I traded my MXT for an XT-70 with a six inch HF coil.
We'll another guy with a MXT got the first gold. It was soooo tiny I was amazed and sad I sold my MXT until I hit an even smaller nugget with my XT-70. Long story short, I took one of the small nuggets I found with the XT-70 and with the other gold hunters around to insure a fair test I tried my F-75 on the last day by laying the nugget on the ground. I like the fast swing and the larger 11" footprint of the F-75 to cover more ground and everyone agreed the F-75 was equal to or deeper than the XT-70 so I used the F-75 and doubled my take of the other days with the F-75. Both machines were running as hot as could be.
I do like the balance of the F-75 better than the XT-70 so once I returned from nugget shooting the XT-70 stayed in the corner as a back up until I sent my F-75 in for the upgrade. During the time my F-75 was away there was snow on the ground so I couldn't use either machine. When the F-75 was returned a trip to the coast availed itself and I took both machines. The F-75 falsed in the wet sand and was as noisy as can be. Something wrong with the coil or something so I switched to the XT-70 with the stock coil. I couldn't remember how to get into beach mode so I picked a pattern, auto ground balanced and away I went. Didn't find much of anything, but the XT-70 was quiet and stable. Later discovered a loose connection on the F-75 and threw some coins down to try it out. Like the MXT, coins didn't lock on but came within a small range. I tried the same with the XT-70 and kaboom, the XT-70 locked on each coin without any movement at all. A solid 30 on a penny instead of a range on the F-75. I was impressed. Now this is MDing!
Unfortunately, the quick fix on the F-75 didn't last and I had to send it back to repair
I still don't have much time on the XT-70, but the snow is gone and I'm getting out some now. The more I use the F-75, the more I'm liking the XT-70. Granted, there is a coil or something problem with the F-75 and I expect it will come back good as new, but quality control seems to be a problem with the F-75. It ticks me that I had to send it back to repair right after getting it back with a clean bill of health. Others report the same thing.
Now to your question. I like the balance of the F-75 better. I like the fast swing rate of the F-75 better. I like the 5 year guarantee of the F-75 better. The F-75 is fast, fast, fast and works well in trash even with the big 11" DD coil. I feel the F-75 is just as good if not better than the XT-70 nugget shooting. The batteries on the F-75 last forever. Even in the limited time running the XT-70 I've changed batteries twice and the F-75 is still on the third set after a year.
That said, I'll be buying a new small coil for the XT-70 before I buy a small coil for the F-75 because of quality issues. My MXT never had a problem, you never read about MXT's having problems. The X-Terra series had some issues early on if I recall correctly, but you don't read much about quality control problems lately. The F-75 however is burdened by love/hate comments. You read where some just love their problem free F-75 or hate their problem ridden F-75. I'm not a very handy man. I like to turn something on... my car, computer, phone, radio, metal detector or what ever and it just works time after time after time.
I don't mind paying extra for quality which is why I chose the F-75 (top of the fisher line) and the XT-70. I mostly coin shoot and once in a while nugget shoot and if I had a problem free F-75 others talk about I would choose it over the XT-70 any day because of my personal MDing style and qualities mentioned above, but until that time I'm investing in more coils for my XT-70 which I'm liking more and more the more I use it. The XT-70 also breaks down into a airline carry on (the F-75 is just a little to big) bag so the XT-70 will be traveling with me whenever I fly (which I do a lot). I don't like to check my expensive machines the way bags are "lost" by the airlines these days.
Right now, because of poor quality control I would say go with the XT-70. However, if Fisher can put out consistent quality F-75's, that will be the machine to beat in the future.
Hope this helps with your choice.
HH
Mark