AT&T. Really no reason to switch. They mostly have LTE here, and the places that don't have LTE still have HSPA+, which I've tested at 8-9 Mbps at times. T-Mobile would be cheaper but their indoor coverage isn't as great. Plus, each of the lines on the account expire at a different time.
Not thinking about getting a 5S yet. No such thing is available right now, so I can't decide if I'd want one. But I like my 4S on 5.1.1, so I may not upgrade