I've had both detectors twice each. As others have said, there are pluses and minuses to each.
Both are very capable of finding deep coins and relics. The F75 likes iron, the CZ-3D absolutely loves it. Longtime Cz-3D users say you can tell iron falses from good targets and that may be true most of the time. But I had enough "that's a lousy iron signal but I'm going to dig it anyway" targets turn out to be good targets that it frustrated me. In clean ground the Cz-3d was a joy to use, in trashier ground not as good.
The Cz-3D is an analog machine and there is a wide variance in depth capabilities from one detector to the next. I've seen Tom calibrated 3d's go from 8 something inches to past 12 in Tom's Florida dirt. In general, the 1021's are deeper than newer El Paso units, but it's not a hard and fast rule. Units that are not properly calibrated are subject to unwanted condtions like incorrect IDing and deep audio smear. Depth differences between individual F75 units are negligable, as digital detectors tend to be. I definitely prefer the sound of the CZ-3D audio to that of the F75.
The F75 is prone to EMI interference, especially with the bigger stock coil, while the 3D is not. A lot of anecdotal evidence points to the more newly manufactured F75's (and newer manufactured high gain FT products in general) being less EMI prone. The F75 is noticeably better ergonomically than the CZ-3D
I've never found anything better than the F75 with the 5" coil for finding coins at depth in dense iron. In a less trashy hunting environment, I would prefer a deep and properly tuned 1021 CZ-3D over the F75.