I really don't think the 5C is ugly or cheap looking, then again I haven't seen it in person, and none of us have, so it's pointless to speculate at this point.
I might keep my 4S until the 6, but if I were to upgrade, I would get the 5S just because of the superior internals. I put cases on my iPhones, so getting a 5C wouldn't really look that different inside a case. If I were to get 5C, I would only be interested in white or black.
The only thing that deters me from the 5C is the fact that it's a first time product (as far as some of the hardware), so the casing might have defects, or something about it might go wrong in the first generation of it's existence...that, and the fact that it's just an iPhone 5 with a different backing.
Again, 5S for me, for it's internals and for the fact that it would be the best most powerful iPhone currently out. No point for me to upgrade to a iPhone 5 with a plastic back (5C).