I’m just testing a scene and am a bit confused about the render times. The latest nightly build (blender-2.79-e59cf13-win64) renders the same scene 3 times faster than the original release. I use GPU rendering in both versions - without the additional CPU checked in the nightly build of course.
Sure, this makes totally sense, because they constantly developing and improving cycles.
You can take a look on what is coming up next here.
The difference between the official versions and nightly builds are, that the official versions are more stable and are suggested to use in productions and with the nightly builds you can test some new features, but they aren´t so stable, but if you use a nightly build and it works for you, go with it.