Unity is generally more mature, which means it's easier to work with out of the box but more choice have been made for you -- including, probably, some you won't like.
Unity has a bigger ecosystem of 3rd party plugins right now.
Stingray is a C++ core with Lua for extensibility. Unity is C++ with C#. I'm not up on the licensing for regular stingray but C++ access to the innards is generally more practical in Stingray than in Unity if you have source access. Personally I prefer C# (when nobody's looking I use Boo instead to axe the curly-brackets) but that's as much familiarity as anything else.
Perf wise I suspect stingray will top out higher. High performance unity requires very careful attention to the nuances of memory management (which many C# programmers don't have). OTOH, stingray being less mature may need somebody who is graphically savvy to get the most out of it. Both engines are really designed around small teams and projects, but stingray handles assets more conventionally so it's more scalable. Stingray does all their intermediate files in a nice JSON variant which is easy to work with using any tools language you like so it's easy to troll the project and find things or make bulk edits. Unity on the other hand has a nice callback mechanism that triggers when assets are imported and allows you to tweak the way they are processed.
Both use FBX natively, which simplifies upstream art creation. Stingray shaders can actually run inside of Maya 2016 plus so it's easy for artists to see something very like the final product without exporting.
Compared to older engines (Unreal or especially Crytek) both engines are generally pretty sane.