Right now I'm using the Edge+ and my partner has the iPhone 6 plus. The 6S plus is not that much different, although heavier. Get the Edge+.
It is by far the best looking of the bunch, especially in Silver or Gold. This isn't even opinion it's just fact.
It's also a LOT lighter than the 6s plus. Like coming from the Note 4 (176g) to this (154g) it's very noticeable. There's a lot less wrist fatigue when using it one-handed, I can't imagine how bad the 6s plus would be at almost 200g.
Wireless charging and fast charging. Enough said. I don't even use fast-charging, I bought myself a couple of Tylt Vu wireless chargers. Look it up, unlike other wireless charging "pads", with this one it's a tilted phone stand, no hunting for the sweet spot. I just drop my Edge+ and it's charging, and with the tilted view I can see when my notifications pop up.
I know TouchWiz lag is all too well-known, but I personally have never experienced it. I've had the S3, Note 3, Note 4, and this guy but all I do is install Nova Launcher as soon as I get them and I go year to year without any UI lag. Not to mention that with the included Smart Manager out of the box, you can basically run the S6 edge+ like an iPhone. Choose and set which apps are allowed to run in the background and you can run a tight ship with your background processes just like iOS does.
Last but not least, better camera. Not to say that the 6s camera is bad, iPhones NEVER have bad cameras and for the longest time it was the king of easy-to-use yet great photos for phone cameras, but not anymore. The competition has caught up on shutter speed (the iPhone's main advantage) and are now just as fast when snapping photos but in some cases, produce superior results. This is exactly what the Edge+ has, a super speedy snapper with more dynamic range, better auto-HDR mode, more stable OIS in video recording, not to mention a lot more zoomability because of more megapixels.
The only areas the iPhone beats the Edge+ is standby battery, slow-motion video recording, OS updates and I guess the app store.
The Edge+ slimmer, lighter, and has an overall smaller footprint while having a bigger (not to mention higher resolution) screen. Android has more customizability, compatibility (wireless charging, Samsung pay) and versatility (Wi-fi hotspots, true NFC) than iOS. Unless you're heavily invested in Apple's ecosystem you will find it very hard to find good reasons to pick the 6s plus over the Edge+.
I'll leave you with a youtube video comparing the Edge+, 6s plus and G4 cameras. Notice how Samsung wins 3 out of 5 categories and yet (shamefully) the testers still declare there's no clear winner in the contest. But unless you're blind, the winner is obvious.
https://www.youtube.com/watch?v=b_1Bgfl6qvk
A much more unbiased and comprehensive comparison of the phones as a whole from Pocketnow here:
https://www.youtube.com/watch?v=PKmmdEtyqvk
Good luck, in the end only you can decide which phone is right for you. Go with whatever feels right.