Theres no advantage to buying at apple.com, both places will have the same prices and youll have to pay tax at both places. At the store youll get it sooner which is worth a bit of money usually.
If you are going to buy online then get it from a reseller like macmall, amazon, newegg, etc. Sometimes those places have specials going on or sales that the official Apple stores never have. Plus they dont charge tax in most states which can save you a ton of money.