It depends on the level of care you're willing to give it.

Carbon steel tends to be easier to sharpen, more durable, and it gains a beautiful patina with age. However, it requires a bit of care to insure it wont start rusting.

Stainless steel is better for use around water, or if you aren't going to give the knife very much care (say, by leaving it in an airplane or boat survival kit year round).