I don't have any experience with Fisher's CZ-21 or Aquapulse, but from what I've read they are quite capable machines, though the CZ-21 is best for detecting in freshwater bodies. I do own an Excalibur, and it is a very good machine, being able to use it in and out of the water at both freshwater and saltwater beaches. It is heavy though, especially felt when working the wet sand for long periods, since it really isn't all that well balanced.
I'm sure others with experience on the other machines will chime in with their thoughts.