From my samples, CZ 97 wins hands-down. I've gotten sub-1" groups with my CZ-97 at my best. At my best with my Sig P220 ST, I've gotten slightly below 2" groups. Both pistols are more accurate than needed for defense, but IMO the CZ 97 has a significant accuracy advantage with most loads.
Having said this, I prefer the Sig for 3 reasons:
1. Superior interior workmanship (in my samples)
2. Superior reliability (in my samples)
3. Better ergonomics, IMO. The CZ has great ergonomics, but it is on the large size.