I think the 603s is discontinued now. I believe the 604 is the same mic but you get omni capsules too (and a pad and HPF).
I like the 603s, haven't used any of the Naiant mics yet (which is pretty disgraceful, I intend to pick some up soon).
If I was in your position I'd try the Naiants because they're cheap, lots of people say they sound good and they're designed, made and sold by a cool guy. If for any reason they're not to your taste you could sell them to me and start sampling SDC's (like the 604).