Well, competition will keep prices relatively low on all of them, although there's no guarantee that they'll be the same price. Amazon, if you're Prime, may cost more because shipping is included. Best Buy has the best warranty, to the best of my knowledge. Moto Maker will keep the phone at cost, depending on the customizations you add like a wood or leather back and memory. For example: I'll be using Moto Maker, and am prepared to spend extra for 64 GB and a wood back. I'll likely get a Squaretrade warranty on it, as there would be no way for Assurion to replace it.
As far as the moment of release? It's anybody's guess. I know I'll be up early-early on the 3rd, checking Moto Maker for the option to order. The advantage of Best Buy is that you can walk out of the store with your new MX Pure: Amazon will take a couple of days to ship, and Moto Maker will take upwards of a week (I project) to build and ship the device.