I'd say unless you're significantly better with the Sig, then buy the CZ b/c it's half the cost, and with the extra $300+ you can buy a **** load of ammo and range time (so you'll be a much better shot).
Sigs are nice guns, but I think CZs are nice enough (and more accurate) to justify buying one over a Sig due to the significant cost differential (unless money is no object to you and you perform better with a Sig).
IMO I actually prefer CZs to Sigs even if they cost the same. I just shoot much better with a CZ. I've shot Sigs, and they're great guns, but they don't do it for me. In the expensive lines, I prefer HKs and perform better with them than Sigs. But, again, Sigs are outstanding guns and considered high end guns for a reason. So you can't go wrong with either one.