It's really a matter of style. With FP-100c, you can shoot with beautiful vintage cameras, like the Polaroid 250. There are a zillion other interesting compatible cameras and devices - I've got a Polaroid passport camera that uses FP-100c and is great fun to take to parties. You can also shoot it in a 4x5 camera with a special reducing back. You can do emulsion lifts and/or bleach out the negative, but these are alternative processes, so don't expect 100% perfection.
I like the look of the Instax film okey, though I think the 100c is a little richer, but with Instax film you are mostly limited to shooting the Fuji Instax camera, which I don't like as much. As another poster mentioned, there are more and more Instax backs and adapters being made for interesting cameras.
Instax film is easier to find overseas, especially in Asia. FP-100c is almost impossible to find anywhere, except specialty camera stores and over the web.
San Francisco, CA