Originally Posted by pauleo

Flax oil contains an omega fatty acid called ALA. Fish oil contains other omega fatty acids called EPA and DHA. Your body converts ALA into EPA and in turn converts that into DHA, and uses the EPA and DHA to make cell membranes. It's very important for health.

There's an argument that some people have difficulty converting ALA into EPA and DHA, and therefore that it's better to take fish oil than flax oil. I never saw much support for that argument though, so I don't know how true it is.