Nexus 6 will be available on Sprint as well, and if you want to buy it from this US carrier, you will have to wait until November 14th. Then, you will be able to get your Google flagship directly from stores or online, but it is yet unclear if you will find it in all the locations.
Sprint is giving you more buying options, including buying the device up front for $696 or on a 24-month plan for $29 per month. Indeed, the device sold by Sprint is the same but more expensive than Nexus 6 sold on Google Play and Motorola ($649).
Remember that Nexus 6 is an unlocked device, which – in theory – means you can buy it from any carrier or electronics provider and use your existing SIM without a problem.