1 gm of krill oil (420mg omega-3) and 1 gm of fish oil (500mg omega-3) is there any different in terms of efficiency and absorption ?

See below: Both are good sources of omega-3's & both should be purchased from reliable sources. Major concern is possible mercury contamination, which is more likely & in higher concentration in fish as they are higher in food chain & toxins are more concentrated. Krill come from deep antarctic waters where toxins are less likely to be found. Sources also recommend astaxanthin (an antioxidant) be present.