IMO, Spriter is better and cheaper though.
It run faster and lighter during gameplay too.
3 features that have Spine and which do not have Spriter:
1) Meshes. Instead of drawing rectangles, meshes allow you to specify a polygon inside your image.
2) Free-Form Deformation. Free-Form Deformation (FFD) allows you to move individual mesh vertices to deform the image. FFD allows meshes to stretch, squash, bend and bounce in ways that aren't possible using rectangular images.
3) Graph Editor. The graph editor defines bezier curves for interpolation between keys, allowing for much more life-like movement.
Spriter good tools, but over the last year, many of my friends went to the animators Spine, including due to the ease of importing animations in Unity and similar projects.