The big problem with the Sony store is that they check the nationality of your credit card. Therefore, if you do not have a US credit card, the only way to buy books is using gift cards (which also have to be bought with a US credit card...).
Barnes & Noble on the other hand don't care about where your credit card comes from. They just do an IP address check.
And let me tell you, faking your IP address is much easier than trying to find some method of payment to the Sony store. I personally use AlwaysVPN (one-off $8.50 payment), which is fast and easy to use. You can buy books through the B&N website and then download them directly to your nook over Wi-Fi (you don't need a US IP address to download books - just to buy).