Black oil sunflower seeds only have trace amounts of Omega 3 fatty acids, which are better for the coat and hoof health. They do have high Vitamin E levels though.
Flax has a lot higher Omega 3s (with a smaller amount of Omega 6s) and has worked a lot better IME for coat, hoof, and skin health. Knock on wood, I have had NO rain rot this year!!! I buy bulk no-name ground flax from my local feed store for $28 for a 50 lb bag. I feed 1/2 cup to most of my horses. 1 cup to those needing extra calories. You can feed up to 4 cups a day safely.
1 cup of flax holds 4 oz by weight. Flax is light weight, so you can feed a higher volume than you can with rice bran or oils.