None of that is true except overnight. I'm assuming you use Best Buy several years ago.
It is 2 days usually and they have loaner smart phones. Usually older ones obviously, the store near me uses stuff like the Evo for Sprint.
Most importantly they don't charge the deductible that Assurion does, though Assurion will cover theft which Best Buy doesn't.
Depends on if you think you are more likely to damage your phone or have it stolen. Damaged Best Buy will save you quite a bit of money, stolen obviously go Assurion, but you will pay more if you drop it.