To be very honest, it's a tough choice. I've gotten so used to the way that samsung do things over the years, and that's why I've decided to change to the G4. So I can still reap all the benefits of Android, but have something fresh and new to look at.
If I had to pick again, between the two, I would pick the G4 at this stage. Probably because it still have a new feel to it. But there are somethings which I have discovered are not in the G4, that the S6 has. But these are just smaller software nuances and can easily be fixed.
One of them, for example, is the lack of complete silent mode. The S5 had this at the beginning of the Lollipop roll out, and was added in a later release. Issue still seems to be present on the LG. (Other G4 owners, tell me if I'm missing something here?)