Flax oil is worthless. The omega-3 content is short-chain omega-3s; your body only processes long-chain omega-3s. Short-chain omega-3s can be converted with great difficulty by the body to long-chain, but at only a 1 to 5 percent efficiency. Take what you think you are getting in flax omega-3s and then take 1 to 5 percent of it -- that's what you are actually getting.
That said, flax powder (not oil) is useful for it's crude fiber content and crude protein content, but nothing more.
Fish oil is what is actually studied and shows benefits in terms of Omega-3s. Cod liver oil is not a good source because it is relatively contaminated as the liver is filtration for the fish body.
You want a fish oil that is PCBB-free and is distilled from the body, not the head or liver. Health from the Sun makes a great, certified product and it only runs about $7 bottle.
Calorie-wise, it's all fat, all 9 cal/gram.