I hate comments like the above. Choices are individual in nature. Contending that a particular course of action is "smartest" implies that others are "dumber," and that's blatantly not true.
I'm on the "S" cycle and I'll get the 5S, assuming I don't go Android (which, frankly, I have been considering for some time). The reasons are severalfold:
• Principle: A big part of that monthly cell bill is subsidization of your device. When you're voluntarily bypassing an upgrade, you're essentially giving your carrier free money. That bothers me.
• Cost: Given resale values, going from a base 4S to a 5S is essentially costless. (This falls apart when talking about larger capacities, which is the boat the OP is in.) Moreover, that trend has continued for some time, meaning I'll likely be able to rinse and repeat in 2 years with the 6S. I'm a fan of that.
• I do see the 5S as a worthy upgrade. Game-changing? Absolutely not. But it satisfies the #1 gripe I have with my 4S and that I had with my 3GS (speed).
• I see the upgrades as generally linear. Non-S models introduce form factor changes, while S models improve performance. I don't think of non-S models as necessarily being substantially "larger" upgrades in and of themselves, although I freely admit that I probably would have preferred to be on a non-S cycle. That said, the difference isn't nearly big enough for me to pursue one of the alternative courses of action to get me onto a non-S cycle, in light of the considerations I raise above.