The latest offerings from Samsung definitely look good, and I'm impressed with how much better they've made their phones since the early days of the Galaxy. I'm still not a fan of their Android skin, but it doesn't appear to look or work as annoyingly as it once did.
That said, I doubt I'll give up my 6P for either offering. I own this phone outright, and switching so soon after getting it may irk my wife. The biggest matter, as others have noted, is the speed with which you can obtain updates with the Nexus line.
My Android lineage has been the Droid Incredible (bought the weekend it came out, switch to Verizon for it), followed by the Galaxy Nexus (Verizon), HTC One M8 (Verizon), and now the Nexus 6P (direct from Google, using it on Verizon).
HTC basically seemed to drop the Incredible about 8 to 12 months after it came out. Updates were incredibly slow to reach the phone, and the last 6 months I used it I'd get low storage warnings when I still had around 6GB of storage left on the phone. Swapped it for the Galaxy Nexus, and boy was that a mistake. Updates still had to be approved by Verizon, and after about a year the relationship between Google and Verizon soured and the phone was like this red headed stepchild. Gave up my unlimited plan to get the M8 on subsidy and really liked it, but a little more than a year in it was sluggish, and updates were still coming in months after the latest version of Android was released. M8 owners are still waiting for Marshmallow, and by the time it hits many early buyers could just get the M10 if they wanted, or jump to another brand.
Since moving back to the Nexus line purchased directly from Google, I've had a couple security updates pushed to my phone without any problems. Stock Android is fantastic, and whenever they roll out Android 7.0 I'll be one of the first to get it.
I don't have a huge grudge against other handset makers, but I want a stock Android experience with updates as fast as I can get them, and it the Nexus line appears to deliver on that better than anyone else.