Yeah, the EPA is the thing that's lacking in flax seed as ALA doesn't convert efficiently into it.
Having said that, there are several fatty acids we need to be healthy. Fish oil gets you two of them, but flax seed's ALA can be turned into whichever one(s) your body is in need of. There are also contamination and toxicity issues with fish oil, both from the fish itself and from the bleaching/processing the oil goes through- though most products are relatively safe. Plus animal cruelty/environmental issues if those concern you.