th_underdog is on the mark.
If one consumes anything similar to the normal Western diet, he's getting far too much omega-6 (linoleic acid) cmpared to omega-3 (EPA+DHA from fish oils being the most "potent"). Thus, supplementation usually should focus on consuming the much harder to find omega-3s.
The "plant omega-3, a.k.a. linolenic acid, does appear to have at least some unique benefits, however and ground flax offers the benefits of fiber and lignans.
PS walnuts, for those who aren't allergic, are also a decent source of (omega-3) linolenic acid.
PPS You might want to do a searchat the top of the page for "Practical Fats" or "Good Fat and Where It's At"