Personally, I'd never recommend anybody buy ANYTHING from Best Buy and God help you if you actually do need to use their extended protection plan.
The Kindles are exactly the same. The standard warranty provided through Amazon is the same. Basically (US Customers and many international) if you need a new Kindle Amazon will overnight you one.
Amazon has a more lenient return policy and process for the first 30 days.
If you really desire an extended warranty then I'd go with Square Trade before either Amazon or Best Buy. I really don't bother though, my Amex card doubles Amazon's warranty and that's plenty for me considering the now lower cost of these items.