What would be the main differences between a paleo diet and a ketogenic diet?

Also, at Price/Costco they have "sales" on Alaska king crab legs. I eat them
with butter, a small cheat, and garlic. Do others on the paleo diet like
crab? If so, you might want to check at your Costco for the dates of these
sales, they seem to come about once a month. Would Alaskan crab be better
for us than the local California crab from off the coast here? (less
mercury?) And is crab in general  considered a high mercury fish?