Ok, I think you're right. I've never bought through Best Buy, so I double checked their info. They give you the option to buy outright and still get the sale price, so it would only make sense that you could get the sale and pay it off early with no penalty like you would with Verizon.
In our case, we are fine keeping phones 2 years or more, so it works out for us to get the extra $100 off on the Verizon promo. I'm pretty sure the trade in credit was a bit more through them. $131 through Verizon, and I think only $100 through BB.