Yes, the G3 is not as tall. But when we are talking about actual USAGE, we must take into account that ultimately anything from the top edge of the screen on upward is irrelevant and has no impact on functionality, ONLY portability and how it fits into the pocket.
Since this thread is about one handed texting and reaching the top corners, we can ignore the top bezel, that can be discussed in another thread as this one is not talking about portability.
Now when we compare the two phones, we see that the iPhone 6 Plus has more usable screen due to the navigation buttons. You can see more in a web article, more tweets, more messages, email, etc. When watching videos they automatically hide. So normal day to day apps, you see more on the iPhone. Notice where the navigation buttons stop (estimated as the home screen does not have the nav buttons encompassed in black).
Now what is important is where the top of the edge lies. The top edge of the iPhone is only a tad bit higher than on the G3. Barely higher at all. And really everything above that top edge is pointless in a conversation about usability.
You're exactly right.