A good deal on a CZ21 is a good deal, period.
I've got an new to me older Excal to add to the 2 CZ's I've got for beach work (CZ-6a that I like to run the FZ-12 coil on and a CZ-20 with the 8 inch coil).
In maybe 20 hours of hunting, I have not found the depth with the Excal that I get with the CZ's. There are hunters who get very deep with Excals, I haven't found the trick yet, even using the same knob position settings others use as their default set up. It does seem to be sensitive to set up, more so than the CZ, and takes a differently tuned ear with different audio clues. Excal runs the DD coil that can give you more ground volume coverage per sweep than a similar sized concentric. CZ's run only concentric coils.
I've got lots of hours on the CZ's and they find things very deep in the sand for me. I need more time on the Excal, but for me, I don't think it will be replacing the CZ's anytime soon.
What ever you end up staying with, I'm sure you know how familiarity and comfort with the detector make a difference. The more you hunt with it and get comfortable with setting it up and what it tells you, the better the combination of you and the detector get. No news in that.
In discriminate mode, set it to 0 and listen to all tones. If you are getting falsing, work the sensitivity down some and maybe slow the sweep some. A low tone breaking or bouncing to high tone is a frequent indicator of iron, like beach tent stakes, nails or hair pins. Slow the sweep way down and see if it stays low tone telling you "iron". A low to mid-tone bounce is a must dig at the beach. Some people say the CZ's love iron ... maybe when you are first learning them. If you run the discriminator above 0 and start discriminating out some or all of the iron, when you get the high tone bounce, you can't really tell if it is an iron bounce or an iffy, deep, good high tone target. That's why I think keeping it set on 0 and listening to all tones is important with the CZ. Once you learn a CZ, you don't dig much iron unless you want to look for iron relics.
Enjoy the CZ-21. I hope you find plenty of good stuff with it.
Cheers,
tvr