CZ20/21 vs Excal, performance wise are pretty much equal. The main differences are as follows:
Silent search in discriminate mode on a CZ, threshold based on the Excal
Concentric coil designs on the CZ, DD coil on the Excal
Manual ground balance on the CZ, auto ground balance on the Excal
3 Tones in discriminate on a CZ, multi tone on the Excal
Rectangular control box for a CZ, enlongated tube for an Excal ( can make a difference when wanting to hip mount as to what feels better to you)
4 nine volts for a CZ, rechargeable system as well as alkaline battery pack ( I believe AA batteries) for the Excal. Both units will not compromise the internals of the control box if there is a potential leak in the battery compartments. The CZ, if memory serves me right, will last longer in terms of hunting time ( 30-45 hours), before a new set of batteries are needed, vs the Excal in terms of time before a charge is needed...but that's just the nature of the beast with re-chargable battery packs.
Both are slower sweep speed units, and built tough ( in my opinion the CZ, especially the older CZ20, is incredibly built and can take a beating and is more durable than the Excal).
Older CZ's do have the rubber pin point button, which can be subject to leakage if not properly take care of. The newer CZ21 does not have that issue.
The CZ and Excal can get the goods, will hunt virtually anywhere, go deep and last a long time if properly washed off and taken care of. If you prefer auto ground balance, multi tones and DD coil design, the Excal is for you. If you prefer silent search, manual ground balance and distinctive 3 tone ID, the CZ20/21 is for you. Having used both for years, I have never found a major depth difference or sensitivity to smaller, lower conductive targets, in wet salt sand or salt water, between the two. Some beaches the Excal had a slight edge, other beaches the CZ had the edge, but nothing where one would outshine the other dramatically.
Bottom line is you won't go wrong with either unit, performance wise, and it really comes down to your personal preference on the features of one versus the other.
My 2 cents