Both detectors will do just fine for what you plan on using them for. Technically, I'm not the best person to give you advice since I do not own both, I only own an Excal II but I'll give you my impressions. Excal II costs more, if you use your machine a crazy amount the cost eventually might even out by how much you'll save in battery money but right off the bat, the Excal II is >$300 more. Having said that, I absolutely love my Excal, I bought it from a well known member on this site and it has worked flawlessly for me and I love how I can just go out and use it, I don't go out enough to truly be one with my machine, but the Excal II is easy to use, from what I've heard though, the CZ-21 is just as user friendly. If you want to purchase a still in great condition CZ-20, you can purchase 2 of them for the price you would spend on a used Excal II.
To be perfectly honest, although some may disagree, I got the general impression back when I did all my research that the biggest difference between the 2 is the price tag. Both run 2 frequencies or greater, both are great machines down to around 12 inches and transition flawlessly from dry to wet to underwater use. You can set a discrimination on either, can control your sensitivity, and if you so desire you can eliminate trash with either one. You'll find members here who swear by the Excal II and you'll find others who swear by the CZ-21, the honest answer is, if you put your coil over a gold ring while you're holding either one, you're most likely coming home with a gold ring.