[Verizon LG G4] Cwey55:
Well to be honest as I write this I'm working on getting myself into the S6 edge + or Note 5 now that they've come out. More on that later.

But, coming from the S4 back in June my contract happened to be up for upgrade on the exact release date of the G4, I would have been totally content to stick with the Galaxy line had they not gotten away from the removable battery and SD card support in the S6, and the battery that is in the S6 really is pretty tiny. That was the deciding factor for me buying the G4 much like you're concerned with. Other than those things the S6/S6 edge would have been perfect to me. I decided to try something new and went with the G4 on the basis they had the removable battery and SD card slot (and the changeable backs as a bonus).

Naturally with there being two years between my S4 and the G4, the G4 is faster and smoother, has a better display, better camera. That's just advancements in technology at work. Down the line we'll look at the same specs as today's flagship phones as minor leagues.

Read around here and just about any phone forums, you'll see the same things echoed to varying degrees:

S6: Piddly battery life, no SD card, no removable battery, great looking screen, fast, arguably best camera, great build with the glass/metal, quick charge/wireless charge built in

G4: Bigger swappable battery, great (larger) screen, fast but some models lag depending on carrier, arguably best camera, nice changeable backs, wireless charge not built in, some models with touch screen response issues

Like I said you'll read the same things everywhere and just have to decide what works best for you. If you're the type that prefers to swap the battery for a quick boost to 100%, don't buy the S6 obviously. If you like the idea of using wireless charging, don't buy the G4 obviously. If you prefer a smaller screen go with the S6, if you want big go G4 (Or the newer Note 5 or S6 edge + of course), if you want to try a new button scheme go G4 (I do like the rear buttons) or if you like traditional side buttons go Samsung. It's a tough call really, you get what I mean, either way it's been years since the S4 so no matter what you're going to get a better phone due to the advancements both companies have made. Compared to the S4 both the S6 and G4 will fly through processing, have better displays, have better cameras, and be more efficient. You just have to decide which features are more important to you.

Now, I know you didn't mention the newer S6 edge + or Note 5 but they've been released since the G4 was released and in hindsight if they'd been released all at the same time I'd have chosen one of them over the G4 and am probably switching over to one of them soon as I can. As I said earlier the only reason I went with the G4 over the original S6 was the tiny battery in the S6, and in the newer two phones Samsung has beefed up that department to match the G4 at 3000mah. So that said, I'm probably looking to get back into a Samsung, because when it was in it's prime, I really did love my S4.

My main gripe with the G4 is the touch screen response, which like I said if you read around you will see it is pretty common and no known fix has been brought to light. It seems pretty hit or miss but it is really frustrating.

Hopefully I helped and answered some of your questions. If not feel free to let me know.