iPhone 4S is the first iPhone to have both Verizon and Sprint CDMA versions available alongside the AT&T GSM version. CDMA is famous for its rock solid reliability and call quality and now that iPhone 4S is a world phone, it can even roam on international GSM network. However, CDMA brings with it some downsides as well, including lack of simultaneous voice and data over 3G, subtle quirks to SMS, and greatly reduced conference call options.
None of these are unique to iPhone 4S on Verizon or Sprint, but they're something to keep in mind if you're thinking of switching to one of those networks now that they have iPhone 4S.
So here's the deal.