Flax = 1900 calories per pound
Rice Bran = 2400 calories per pound
Even so, the digestibility comes in to factor and the fat content. I don't know WHY I've had better luck with flax, but I have.
Based on this, there are 2,110 calories per-pound of Flax. Calories in Flax Plus - Brown FlaxSeed Meal
If I use this one, there are 2,400 calories per-pound. Calories in Ground Flaxseed - Calorie, Fat, Carb, Fiber, & Protein Info
The little calculator on the right of this page says 1 lb has 2,400 calories. See How Many Calories in Cold Milled Brown Flax Seed & More at FitClick
As for fat, most of the above sources agree that around 75% of the calories from flax are from the fat content.
For rice bran, this web site says there are 1,400 calories in 1 lb. See How Many Calories in Crude Rice Bran & More at FitClick
For this particular brand, there are 1,800 calories in 1 lb. See How Many Calories in Bob's Red Mill Rice Bran & More at FitClick
This one, using the 1 ounce drop-down, says there are 1,400 calories per-pound. Nutrition Facts and Analysis for Rice bran, crude
This horse nutrition site says it has about 1,200 calories per-pound. Rice Bran for Horses
The fitclick web site says about 40% of the calories from rice bran are from fat.
So, it looks like flax is higher in fat per-pound and higher in calorie. It's also higher in Omega 3 fatty acids, which are more beneficial in a horse's diet and more deficient than Omega 6 fatty acids. Rice bran has some Omega 3s, but not nearly as much as flax meal.