For starters, these are unlocked international units. This is important to know because it means you won’t be able to use it on carriers like Sprint and Verizon, though using it with GSM-based AT&T and T-Mobile should be just fine. This also means your device doesn’t come with a manufacturer’s warranty — this is because they aren’t actually intended to be sold in the US, and thus Sony can opt to refuse support for the phone in the event that it malfunctions.
That said, Expansys is providing the option to go with their own service warranty, so that might be one slight reason to go with Expansys over Amazon. Otherwise, prices range anywhere between $700 and $780, depending on color choice. Be sure to explore the listings of both marketplaces and make a wise decision before plunging in.