I know that there were some places selling it on sale for $40 or less. So in that case, it makes more sense due to the $10 savings.
I bought it off Steam since it was pre-purchase for $45 and mostly to play the TF2 beta weeks early which was essentially the full game. And I don't pay sales tax, so unless it is on sale in retail (and it won't be for me as I only have Walmart to buy from), Steam will be cheaper.
Normally I advocate Steam purchasing, and it's been great for me, but with TOB, you might be able to pay less retail, and if not, roughly the same. Of course, it if requires a fairly long drive or amount of time, Steam might be better. But more than likely, you can go to the store to buy it, come home and install, update, and be playing well before it is done downloading. And since TF2 is so damn good, I suggest going to the store NOW so you are playing ASAP.