There are two currently available versions of the S4, the standard and the Active. The active is waterproof, to a point. Some don't like the compromises of the active, such as all controls being soft. There is a mini version and a camera version coming. The differences between the carriers are their network coverage, their rates, their policies, and their technology. For instance, AT&T and T-mobile are GSM, while Sprint and Verizon are CDMA. Verizon has the best overall coverage, but it does vary by area. Sprint currently has unlimited data, but that could change. Verizon has the most extensive LTE (4G) network, but the others are catching up. Where I live, Verizon has the best coverage, so that's who I have despite the limited data plan.