Where's the best place to buy an iPhone (6)?

So I've been trying to plan my attack for awhile now and lately I've been wondering:

Where's the best place to buy an iPhone? I will have cash for the full-value and most likely get AppleCare.

Here's several options and pros/cons that I know of:

T-Mobile Store: I could do it via an EIP and only have to pay a small amount each month.
Apple Store: It's the Apple store.
Best Buy: I've heard their personal warranty plan is even better than Apple's? Is this true?
Costco: I've heard you get free accessories, etc. Is this true?
Target/Walmart: Are there any benefits here?

So, is what I've heard true? Where's the best place to go?