Hi Spanky,
The answer is pretty involved. It was quite a debate for a few years too, but the more recent fish oil studies have pretty much ended the debate. Flax is a great addition to any diet, but the primary Omega 3 it contains (ala) has to be converted to the primary omega 3s us humans need (epa and dha). The conversion is very poor. Fish oil contains very high epa and dha thus conversion is unnecessary.
Geeky stuff
The big issue in the primal/paleo community is omega 6/3 ratios. On the SAD (standard american diet) we become saturated with polyunsaturated omega 6s. In abundance they are highly inflammatory; perhaps worse than fructose and as a bad as grains. Refined seed oils are pure omega 6. The PUFA (polyunsaturated fatty acids) in most of our meat is omega 6. This is one reason for the emphasis on saturated fats and pastured meats, to reduce the inflammatory O6s. Just as important is the addition of anti-inflammatory omega 3s. Eat fish a couple times a week.
The 6:3 ratio in most standard diets is between 20:1 and 12:1. For optimum health and to stop inflammation preventing a long list of autoimmune issues and accelerating healthy weight loss, it's widely accepted that a 4:1 ratio or lower is correct. So this requires cranking up the 03s as high as comfortable and eliminating unnecessary sources of 06. I think a lot of primals and paleos get the grains and sugars right but don't really bother with all the PUFA concerns and continue to use corn and canola oils at home and don't hesitate to stock their pantries with soy, cottonseed, and safflower oil based products. Eliminating them is as important as avoiding white bread and Coca Cola. It's also a huge challenge.
