They two films are quite different. As I'm sure you know, the FP is a peel-apart film, while Instax is integral. I find the resolution and image quality to be quite a bit better with FP-100C. Even with a camera as primitive as the Polaroid Big Shot, I get astonishing (to me) results, and obviously even better if I'm using it to proof with a Hasselblad and Polaroid back.
But I love the Instax, too. I have a Wide and two of the Minis (got the second one at a stoop sale for $5 with a full pack of film--couldn't pass that up). They are great for birthday parties and other get-togethers where people can pass them around. Kids are fascinated by them, too, as the image fades in.
I think it's a great gift to give, though it's one of the ugliest cameras I've ever seen. It's also really big. In fact, depending on what it's used for, it can be more "fun" than an old 450 loaded with FP-100C. Also you don't have to throw away as much stuff when you're done shooting. The Instax Mini is another great gift.
