There isn't really a right or wrong answer. It's all a question of relative value.
In a years time, you probably won't remember what you spent the 20 dollars on that you will save if you purchase from Amazon, but at the same time, in a year, the two week wait for delivery will have ceased to be an issue as well.
Some people would recommend, if you play games at your local game store, that you buy your product there, and consider the extra 20 bucks a delayed entry fee for all the free game time you've at the store.
(I ordered from Amazon. I don't have a pressing need for the book this week, and I don't play in any of the local organized games, so no reason for me to pay more. The twenty dollars was more valuable to me than having the book an extra two weeks.)