Krill is further down the food chain and has less toxins.

The omega is also bound by phospholipids, which presumely should make it more effective.

Personally I do believe fish oil is the best choice, because of higher levels of EPA/DHA.

Veganism, vegetarianism mostly. Fish oils are now certified independently by IFOS who have an excellent site showing actual tests of retail fish oils, not sure if I can link to it here but searching for "ifos certifications" will bring up the site. Mercury is not an issue for certified fish oils.

I believe it's significantly easier and more space efficient to farm krill because of their size.