You shouldn't buy the iPhone 5s right now. Not with the iPhone 6s launch rumored for September and not with a smaller 4-inch iPhone 6c possible for September.
If you're looking to buy the iPhone 5s, it's likely for one of two reasons. One, it's cheap. Two, you like its 4-inch display.
Well, if you wait until September, there's a good chance that the iPhone 5s price will get a whole lot cheaper. Maybe even free on contract. There's also a chance that we'll see a new 4-inch iPhone, a device that would likely take over for the iPhone 5c and perhaps the iPhone 5s as well.
There's been a ton of back and forth about an iPhone 6c in recent weeks and we're still not sure if it will be coming to shelves this year. That said, if you're even remotely interested in a cheap 4-inch iPhone 6, it's probably worth holding out, even if it's just for more concrete rumors to emerge in the weeks ahead.