I assume you mean Unity's built-in 2D Animation system.
In general the Unity 2D animation system will most likely lead to equal or better performance, since its animation featureset is very limited and uses GPU skinning. This is planned in spine-unity as well (under this issue ticket), but we did not yet get to implement this feature. spine-unity will currently perform CPU skinning (a mesh is generated and rendered using a standard MeshRenderer) but supports a far larger feature set.
You can download the spine-unity runtime free of charge for performing some benchmarks and performance comparisons on your machine.
Spine Unity Download: Download
The main question is if animating and managing skins, constraints, and so on in Unity's 2D animation framework is something that you want to use as a developer or animator. Both animating and managing skins is, well, "not very mature in terms of usability" to put it mildly.
