Always use your Costco Amex card (or another Amex card tha doubles the
warranty) for appliances (no matter where you buy it) if you're not
going to buy an extended warranty since at least you get a second year free.
Also, I know for computers, the extra Amex year of warranty is in
_addition_ to the two year warranty you get from Costco, even though the
second year of that warranty is technically not a factory warranty (Amex
says that they double the factory warranty to a maximum of one extra
year). I don't know if this also applies to appliances, and trying to
get a straight answer out of Amex is darn near impossible. I found that
Costco was significantly more expensive for the same appliance than
Sears, to the point where the extra year of warranty was not worth the
extra cost.
Last two dishwashers I bought I got at Sears. They were about 20% less
expensive for the same model at Home Depot. However if you tend to buy
extended warranties on appliances then Sears is not a good place because
they charge significantly more for an extended warranty than Home Depot
charges, negating the price advantage of Sears.
I'm not a fan of extended warranties, but appliances seem to have become
significantly less reliable and significantly more expensive to repair,
so buying at Costco with an Amex card, if you do get essentially a three
year warranty at no extra cost, might be worth it.