Amazon: You get ten dollars plus a golden pistol and rifle! The golden guns increase your fame in the game, which gives you more things to do, only faster.
Gamestop: You get a fancy outfit that gives you improved dead eye targeting. If that interests you, go for it! Be aware that you can just unlock similar costumes in the game though.
GameCrazy.com: If you ever get stuck, you can just go online and get help. There's no need for an incomplete guide.
RockStar Warehouse: Personally, I think getting a Red Dead Redemption shirt is pretty freaking cool, especially considering how awesome the game's artist is and I imagine he's had some role in the making of it. If Amazon's offer doesn't interest you, I'd go here instead.
Overall I'd say Amazon, you get WAY more bang for your buck. If you don't usually shop there, now would be a great times to start. They offer the best prices on a huge selection of goods and their customer service is phenomenal.