.tscns are "Text Scenes", meaning they're written in a human readable format. You can open them in a text editor to see. They are also friendlier to version control, so you can have meaningful diffs if using Git, Svn or something of the like.
.scns are binary scenes. They tend to be smaller and faster to load. If you have a huge scene (like in 3D with a lot a geometry data) it's the best format to use.
.tscn is converted to
.scn at export time, so you won't lose performance in your game release.