Why would someone choose a Nest over an Ecobee smart thermostat?
On the downside it appears to be missing some of the auto features the nest has, such as the motion sensors(which is my favorite part of it). And the cheaper version coming out is missing even more features than its more expensive older brother. The nest has a very streamlined interface, and is meant to be more of a passive experience, requiring less interaction with it, with more auto adjusting features. It's meant more for a person to set it and let it adjust to you, not requing you to fiddle with it if you don't want, while still having it try to adjust to best fit your energy needs.
They both have pretty good remote apps and algorithms to try to make your home as efficient as possible. They both take note of the weather, and connect to most home heating/cooling units (although the ecobee being around longer works with pretty much all of them). They both can have multiple units talk to each other, and work either in tandem, or individually (zoned). I did my own review of the nest on my site - tasel.wordpress.com/2012/04/03/my-first-month-with... if you want to take a look.