I've got a network unlocked note 4 for testing, with a beta lollipop build. As it is carrier free there's no junk. And it does NOT lag. 3gb ram. No lag. At all. The camera, coupled with ois and the new camera algorithms is great! And raw shooting is an excellent addition.
Our sony z3 beats it in low light, but the overall compression renders the sony pics of a lower quality (fixed with hyperx mod and raw shooting).
The 1020 has the best camera. Bottom line. No ifs ands or buts. It is simply the best. However it is slower than the android flagships, and lack of sd might be a pain. The phone itself is not as good as it was, though wp works well. If you're coming from android you've probably done your homework so you probably know what you're getting and also how much you're losing. Android has some excellent camera phones and excellent all rounders.
But bottom line, the 1020 has the best camera, but the speed is the only thing that lets it down. If you have kids and want to take action shots, it can be a pain. But, my word, when you get it right the pics are remarkable. It is an astonishingly good camera setup and deserves the kudos it gets. It trounces everything. If you're ok with its foibles.