You said you mainly hunt schools, parks or saltwater beaches, a little in shallow water and that leaves me to wonder what you're after other than coins and gold & silver jewelry?
In conditions that can vary from iron mineralized ground to low-conductive salt water beaches, you're going to be working in conditions that can certainly play tricks on the audio and visual Target ID of most any detector. This is especially true when the target gets deeper or at an odd angle, and that's a very common thing when beach hunting.
You started by saying you have a CZ5. Great detector, very well balanced. Quite frankly, I feel that a good working CZ-5 was the best of all the CZ series. They can handle most salt water environments, and they are capable of very good depth. Now, I will argue the "well balanced" comment as I think they are a bit nose heavy, but there are some worse out there.
Your statement that the CZ-5 doesn't separate pennies, dimes and quarters suggests to me that you would like to have better "cherry picking' Target ID info. That makes sense for hunting park and school lawns, for example, but only if you're really going after the more shallow coins. Those in the surface to 4" range because, depending upon conditions, TID will often be errant beyond that depth.
You also said you're considering the MXT, M6, X-Terra 70. F-75 or F-4, and implied that this would be a replacement for the CZ-5. Well, I don't think that's such a good idea because none of those will really work the salt water beach better than a CZ-5, from my experience.
I haven't used the F-4 yet, but I don't think I would be all that excited. I would consider five moves for you to consider, and explain why I would go this route. They are NOT in a particular order that I would chose, however.
#1.. Keep the CZ-5 for beach hunting and deep coin hunting and get a Garrett Ace 250 for working playgrounds and some of the parks and schools where you are mainly after the quarters and dimes.
#2.. Keep or sell the CZ-5 and get a White's IDX Pro, either already modified or have it done so.
#3.. Keep the CZ-5 for beach work or part with it and get a White's XL Pro (or 6000 Pro XL) for better Target ID when cruising the parks and schools.
#4.. Keep the CZ-5 for beach work and get a White's XLT for parks and schools. This will provide you will a lot of visual and audio TID information (which it seems you want) and reasonable performance for 'coin plucking' those sites.
#5.. part with the CZ-5 and get two models. A modified IDX Pro and either an XL Pro or XLT. These tow will cover all your bases, and they share the same search coils of which there are a lot to pick from.
If it were up to ME, I'd go with #5. But, if you're really interested in buying a new, current production model, then of those you mentioned I would go with the Matrix M6 and keep the CZ-5 to compliment it.
Monte