If the ISO (sensitivity) of the 5S's sensor is the same a the 5, then we can answer the question about how much light you need.
ISO is equivalent to how sensitive the film is on a film camera--how "high speed" it is. ISO 100 to 200 is a full stop worth of difference. 200 to 400 same, and so on.
For digital cameras, their sensor has a certain sensitivity, depending on pixel size, sensor size, type of sensor, etc. This is why a higher mega-pixel rating is FAR from being all you need to know about a camera. A higher MP with a lower sensitivity and a smaller aperture = a WORSE CAMERA, no matter how many more MP it has. Apple KNOWS this, thankfully. So the 5S camera sensor sensitivity almost certainly will be at least as good, I suspect, but it may be even better. In which case, the combination of increased sensitivity and increased aperature size might add up to even more than a half stop of difference.
In any case, even a half-stop is significant when you're down in the low-light threshold of your equipment and are holding it by hand. I've been using a dSLR for years, and for the first two I was using the kit lens and wondering why in the heck I couldn't get decent pictures indoors without using the flash. THEN I discovered prime lenses. I got a 50mm f/1.4 and was blown away by what I could do. Last year I added a 35 mm f/2.0 for the wider field of view. These aperatures combined with an ISO of 800 or 1600 allow me to take nice bright photos INSIDE in winter in a not-so-bright house. I took a bunch of photos last Thanksgiving on an overcast winter day and they came out great. Lots of Bokeh (out of focus behind subject) and a little bit grainy, but still quite nice, and no flash artifacts and harshness.
I'm told by my professional photographer friend that I need to learn to use a flash properly, and to invest in a decent flash--and I'm sure she's right--but I still won't ever go back to a lens with a small aperture, like f/3.2 or something. Not unless I get a new dSLR with a lot more sensitivity (i.e. can go to higher ISO).
Anyway, this is why the iPhone cameras are so good. Apple doesn't design them based on just a single parameter, like mega pixels.