Gelcoat is cheaper and will color match a white hull. Epoxy is stronger and more durable, but you'll probably want to tint it to a contrasting color, as it will be slightly off-white if you use white tint. Combined with multiple layers of fiberglass tape, it will make a very rugged keel strip. Another option is to add graphite to the outer layer, which adds lubricity, but results in a charcoal-gray color.