Spine does class/inheritance closer to the "JavaScript" way where properties are resolved correctly at runtime, Backbone seems a little more hackish in this area IIRC (+1 Spine).
Spine seems to have given up the separation between Controller & View. Backbone has a great separation here that has been a great aid to me in refactoring a large application. (+1 Backbone)
Backbone treats collections of Models (Backbone.Collection) as proper first-class citizens. Collections can receive & emit their own events, handle their own serialization/fetching, and make use of all Underscore.js methods (+1 Backbone)
Other than that they seem pretty similar to me. So my verdict: Spine +1, Backbone +2 :)