(basically the 5C is the color you want whereas 5S is not the color you want)
The answer should be obvious -
5 C
5s still has the old look from last years phone and more importantly it doesn't have the color you want
5c is a fresh phone that has a new form factor and has exactly the color you want. Plus the hard plastic back should also be more scratch-resistant.
I say go with your gut feeling and don't listen to dudes who need everything in black & metal to compensate for low self esteem.
You want to get a gift. Imagine your special someone opening your gift:
a) ohh, looks like last years phone but with much more power (5s)
b) wow, a brand new looking phone but only few micro spec-updates (5c)
Performance wise I bet the average user won't notice any difference.
If snapshots are important to you the 5s has an improved lens and will therefore most likely produce better shots.
The bigger issue here are privacy and security flaws which already prompted government officials to warn potential buyers not to trade their biometric data (aka fingerprints) simply for comfort. Unlike a password your biometric data can't be changed. No one can claim that your data will be 100% secure - because there is no such thing. For me this was the No.1 reason not to buy the 5s (next to it's somewhat outdated appearance).
Apple is all about great design not spec sheets - so currently I don't know a better iPhone than the 5 C.