Purchasing through Amazon will net a code that you can immediately plug into your Xbox 360 or Xbox One, while going the Best Buy route means you'll have to wait for a physical card to be mailed off. Or you could simply walk into your local Best Buy and pick one up, granted supplies aren't limited. There's no telling how long subscriptions will be on sale for, so interested parties will want to pounce sooner rather than later.
Xbox Live Gold is necessary for most premium apps, such as Netflix, on Xbox Live as well as for online multiplayer gaming. Having a Gold subscription also guarantees you free games each month. In January, Microsoft hooked up subscribers with free downloads of Sleeping Dogs and Lara Croft and the Guardian of Light.