My rule of thumb with Apple products (especially if you plan to keep them for more than a year) is to get the newest version you can afford. That way, you'll continue receiving support updates for the longest time and you'll have the fastest hardware, which will continue to be able to run updated software versions.
The 5S has some significant hardware improvements over the others. If you're going to keep it for a while and have the money, I'd go with it. The 4S is now 2.5 (I think) years old, while the 5C is just a repackaged iPhone 5 (1.5 year old).
I don't want to get stuck with an old operating system with app upgrade issues in a year.
Apple plans obsolescence into all its devices, but they generally give them a 2-3 year lifespan.