I like your thoughts on both! Very well said.
Just gonna toss my 2 cents on 2 key points.
1) I found that the S6 has a slightly warmer tint compared to the G4. On white images, the S6 has a slight yellowish tint while the G4 looks a little bluish. Both are really excellent displays and the S6 wins the display race to me due to the very deep blacks, but the G4 is a very, VERY close second, as its IPS LCD is almost AMOLED-like in terms of saturation and contrast. A very impressive IPS panel.
2) The performance hiccups you described with the S6 may be due to Samsung's overly aggressive RAM management. The phone has 3GB of LPDDR4, almost exactly like the G4 (I said almost because the G4 uses 3GB of LPDDR3). However, the phone seems to act like it has 2GB of RAM instead. That's why apps stored in memory tend to reload from the beginning quite often and yes, there is the home-screen redraw that occurs if there's not enough memory.
There's no such issue on my G4. There hasn't been any home-screen redraw yet and only apps that have been stored for a really long time had to be reloaded. When they did reload, however, they were back to where I left them, which is very handy.
The S6 has the power, but the overly aggressive RAM management is an issue. No word on when a fix will be issued, but it should be in the next software update. I initially thought this was a Lollipop issue, and initially I was right, but after seeing the HTC One M9 not suffering the same issue, it's definitely a Samsung issue.
Oh, and both phone cameras are superb and the very best I've used, but for casual users, I'd pick the S6 because its Auto mode is just a bit better. The G4, however, is an excellent photographer's companion.