Is all fish oil the same?

I was on fish oil supplement for some time but stopped taking it because it made me nauseous and I tasted fish after wards. My doctor told me I could switch to Flax Seed Oil supplements and I would get the same benefits. Now, I'm finding out that not just any oil will do and while Flax Seed is good, nothing compares to true fish oil. Is this true? If I need to switch back to fish oil, do you know of one that will not make me nauseous or cause me to burp and taste fish after wards? I've read some really good things about Krill Oil, what do you think of that? Any advice or suggestions would be great. Thanks!