Thanks pxlweaver.
First I would say being integrated into Unity could actually be a disadvantage if the integrated tools are less powerful. I don't know if this is the case with Smooth Moves, which I know nothing about. When comparing Spine to other tools, I think the best thing you can do is build an animation in both. Be sure to refine your animation as you would on a real project. Good animation requires a lot of tweaking to get the movement and details necessary. This is where many tools start to fall apart. You want a tool that not only can animate your images, but provides a good workflow. Without actually using both tools, it will be hard for you to really get an idea of how you will integrate them into your workflow.
I'm not familiar with Smooth Moves, but I'll take a quick look at some YouTube videos and ramble off some thoughts. Keep in mind I can't possibly help but be totally biased and these notes are the result of a few minutes of research without actually using Smooth Moves, so I could quite easily be wrong. ๐
Setup is a bit different. Spine has a setup pose (aka bind pose) which all animations are relative to. This enables some easy procedural animation. Eg, leaning the bind pose forward while running up hill will cause the run animation (and other animations) to lean forward. Smooth Moves doesn't do this, each animation defines the absolute pose (you make a pose and copy it as the basis for each new animation).
There are some other setup differences. In Spine images are placed on bones. In Smooth Moves each image has an implicit bone which originates where you set the pivot point. Setting the draw order in Smooth Moves is assigning a number to each image. Spine uses slots to control both draw order and the images that can be attached.
Spine has skins, which let you reuse animations for characters that use the same skeleton but look different. Spine skins are not just a different set of attached images, they handle the problem where animations have image changes. Eg, your character blinks so the animation changes the eyes. If you change the skin, in Smooth Moves (and similar programs) the animation will change to the wrong eyes. In Spine it works (see the goblins example and the skins video).
Smooth Moves suggests changing the material to another with exactly the same layout, which is very limiting. You will very, very often want images in different skins to be different sizes. To have a different skin for the same skeleton in Smooth Moves you'd have to swap each individual image. This would not solve the problem of animations that have image change keys, so for each skin you would need a duplicate animation for those that have image keys.
I didn't see advanced tools like inverse kinematics (the Pose tool in Spine) and bone/image compensation.
Spine has a nice runtime API for loading data, keeping stateless data separate from stateful data, applying animations, mixing animations, manipulating bones for procedural animation (examnple), etc. From a quick look at Smooth Moves' API, procedural animation may be difficult. I see you can get a bone's transform, but it isn't clear how manipulating it will interact with animations that are applied.
From my quick look, the Smooth Moves tools don't look as easy to work with. It seems the Smooth Moves can indeed animate your skeletons, but the workflow is quite different. You'll be spending a lot of time building animations and integrating them with your game, so the entire workflow from start to finish is very important.
Anyone else can feel free to jump in with additional comparisons. ๐
You can see some videos about things I mentioned in Spine here:
http://esotericsoftware.com/spine-videos/