What is the wave action and target density (trash and good targets) like where you hunt most?
My 2 cents: I have an 8 inch coil on my CZ20. A while back, briefly had a 10.5 that I used on a CZ6A in the real shallow stuff (ankle to calf deep). In the wave breaks, the drag on the 10.5 inch coil is a good bit more than on the 8 inch. The thick profile of the coils doesn't help. For me there is plenty of drag on the 8 inch coil. If I were only hunting protected bays and inland lakes with little to no water movement, I'd consider a 10.5 inch CZ21. If you ever hit a trashy area, the 8 inch is a better choice than the 10.5 inch. Depends mainly on your hunting conditions. I lean towards the 8 inch.
Cheers,
tvr