For a field gun, I prefer IC/MOD. WS1 is old technology and is actually reverse (bell shaped) choke that is reputed to throw pretty spotty patterns beyond 20 yds. Serious SxS clay shooters order their CSMC 21's with Brileys. If I need more "scatter" for early season grouse in heavy cover, I use a spreader load.

A Springer Spaniel, a 6# double and a fair day to hunt.