Rice bran and flax seed are similar in that they both provide fat to the diet, but diverge after that point.
Flax provides:
1) an excellent source of supplemental protein
2) fiber from the seed hulls
3) Omega 3 and 6 essentially fatty acids
4) may provide anti-inflammatory properties - good for horses with itchy skin or arthritis (provided by the Omega 3's)
5) May provide some "mucilage" (similar to SandClear/psyllium products) when fed whole
Rice Bran provides:
1) provides some protein
2) excellent calorie source
3) provides Omega 6 essential fatty acids
4) contains gamma oryzanol, a substance also known as "Mother Nature's steroid", which is felt to provide more muscle bulk - hence the reason why a lot of halter horse people use it (though some refuse to use it, claiming the weight goes to the horse's neck)
5) most horses love rice bran and it often times tempts finicky eaters to try something they don't care for.
Both flax seed and rice bran have an inverted calcium
hosphorus ratio, just like grains, i.e. they have more phosphorus than calcium, so should not be fed in large amounts without taking the rest of the dietary balance into consideration.
Robin C