They're quite different but, well, Mercola also just sometimes makes sh*t up making it difficult to separate the fact from his own fantasy.
Krill oil is really expensive and some of us need more Ω3 than we could possibly afford to take in the form of krill.
Also, krill has some pretty powerful hormonal effects, which is good in moderate doses....it increases progesterone which is a powerful steroid hormone neuroprotectant for both sexes and for women, a powerful hormone that maintains mood stability premenstrually. Many women who take NKO continuosly will experience breakthrough bleeding midcycle due to high progesterone levels - other than it being a hassle, I'm not sure that it's a problem.
Take it if you can - or take *some* anyway but chances are you'll still need fish oil to bring Ω3/6 ratios to par.
My strongest recommendation though would be to not buy NKO from mercola. There are only one or two companies processing krill oil in the entire world - they just label it for everyone who sells it. Buy as cheaply as you can per mg of the oil (which will not be Harvest Health....lol, as much as I love them) So far, I've found nothing cheaper than iherb.com.