Please name reasons. According to the guy at the Verizon store, he had both and couldn't detect a difference, although one processor is supposed to be faster.
The S has a better camera, and better flash for the camera, but I don't care about that. The S also has the fingerprint thing, another feature I don't care about.
The C, which I have, is slimmer and I prefer the plastic back, vs glass, which my last iPhone had.
I have zero regrets getting the C.
We have deployed 4, 4S, 5, 5C and 5S phones at my company. I am the IT guy and set up said devices for people, fix them, replace them, etc. There is very little difference in practice between the 5C and 5S. The S is slightly faster (I notice it), but I don't think it will matter much until future versions of IOS come out. That's when processing power will really stand out as newer (read heavier) operating systems come out. The C lets you show a bit of style and does seem to hold up better to drops than the S since there is less glass to break. The thumbprint sensor isn't bad, but it isn't necessary either. It also doesn't work all the time, we had several that had to be replaced for that reason.
As far as cases I HIGHLY recommend against the Otterbox Defender. We deployed hundreds of them and they have the following problems:
Don't protect worth a crap, we have had at least 30 phones with broken screens/back glass after being dropped in the defender. At least 80% of these were people that I trust and they didn't drop it from greater than waist height
The don't prevent dust/sand/water/dirt from getting in at all
Dust/sand/dirt that gets in doesn't want to leave. It just stays there rubbing your phone raw
Rubber outer case stretches to become a wobbly annoying mess if you remove it more than half a dozen times to resolve the aforementioned sand/dust/dirt problem.
Turns your pockets inside out because the outside case is so sticky.
Flaps are annoying as hell. The lifeproof flap opens and stays out of your way. The otterbox flap's job isn't to keep dirt out, it's to keep your charging cable out, and it does it's job quite well.
If you work in a harsh environment then the LifeProof cases can't be beat. They have downsides too like muffling the sound and making it harder to insert/remove the charging cable, and being stupidly expensive, but they will keep dust/dirt/water out and we haven't yet had a phone break in one unless it got run over by a backhoe. I've used one on my company phone for over a year now with no problems. I even dropped it off the top of a 12ft ladder onto concrete and it was unscathed. I admit that what most likely saved it was that it hit one of the rungs about halfway down, but I was impressed all the same.
Just my 2c based on several hundred phones 'in the wild'.