So I went and had a look.
SquareTrade $700-999.99 3-years = $109.99
Costco ST $500-1000 3-years = $59.99
So a $50 difference on the three year warranty.
Assuming you bought the TV from Costco, they have their own contingency which would take place before the ST only plan, but would run parallel to the manufacture's warranty. So that would get you 5 years of protection where SQ only would get you 4.
If you are buying the TV elsewhere and only planed on purchasing 3 years then it appears that getting the ST warranty through Costco is the better deal.