I remember reading a study that proved fish oil was much better than flax oil. It said ALA, the omega 3 found in flax seed is not usable by the body and has to be converted into EPA and DHA. The percentage that was actually converted was quite low. On the other hand, fish oil contains EPA and DHA so nothing needs to be converted and all of it can be used.
Im sure a quick google search can also give u an answer