As far as performance, you can't go wrong with either. But there are differences in each unit.
The CZ is a manually ground balanced unit with a concentric coil design and 3 tones for target ID. It takes four 9V batteries and is in a rectangular type case. Stock configuration makes the unit a bit top heavy, especially if it has the bigger 10.5 inch coil on it, but hip mounting is pretty easy with the hip mount attachment.
The Excal has automatic ground balance, uses a DD coil design and has numerous tones as target ID. It has a rechargable battery as it's stock configuration and is in a long, tubular housing. Most feel that the stock configuration is not that desirable and get a straight shaft for it which is an additional cost. The hip mount config is a bit more cumbersome than the CZ21 in my opinion.
What you prefer in terms of functions may be the deciding factor....the performance, depth, durability, etc. are pretty even. Also, check out the warranty info as that may also factor into the equation for you.
Bottom line is, only you know what you prefer in terms of what functions/features a detector has that is important to you. I would not worry too much about what unit does better because as Joe (oldbeechnut) stated, you can do well with either once you learn whatever unit you get.
