How many amps should an iPhone 5s car charger output?

When looking on Amazon for a car charger I noticed that different items have different amp values, 2.1 and 3.1 being the most frequent. What is the recommended value?

Does it even matter? Is there more to it than the fact that more amps mean faster charging?

More amps will not mean faster charging. The iPhone (and any electronic device) will only take as much current as it requires, and no more. The iPhone will take 1A to charge, and an iPad will take 2.1A.

There's no harm in using a charger that is capable of providing more current than a device requires, but there's no benefit either.

Providing less current than the device requires will lead to longer charging times, or no charging at all.

I disagree with parts of Nathaniel's answer. The normal iphone charger is 5W and just over an amp. Using an iPad charger, either the 10W or newer 12W will absolutely charge your iphone faster. In fact, almost twice as fast. However there's some speculation that it may shorten the battery's lifespan. I've had no problems using an iPad charger on my 5S and it's charged in no time. So, you take a chance I guess but yes, the 2.1 amp ipad charger will cut charging time down considerably.