My wife has the 6S+ and doesnt use her iPad as often, but the Plus hasn't replaced it. It doesn't seem as if she uses the iPad as much for email and browsing the web and social media. The 6S+ really shines there. But when it comes to binge watching Netflix or Amazon Prime Video, the iPad is still king (or queen) for her. The resolution is nearly the same, but the added size makes for a better viewing experience.
For me, the 6S+ is way too big and cumbersome. None of the trade offs in screen size, better battery life, and OIS on the camer are worth it. I can use it one handed, but not easily, and the device feels like a tank to me. The battery life is excellent, but I'm nearly always around a charger, at home or at work or in the car, and so that's not an issue for me. And I'd rather use my iPad for watching video and playing games than my iPhone. So for me the 6S size is the perfect trade off between screen size and still being able to comfortably use the device one handed.
But I realize that what is best for me isn't what's best for everyone. Only you can make that decision for you. If you're tempted to upgrade, go the the Apple Store and pick up a 6S+. You have a 14-day return window if you don't like it or it turns out too big or kills your iPad provided the device isn't damaged when you return it,
Also, there's a rumored updated 4" device coming out really soon (there really are too many rumors out the for this not to be true), so if you can wait for a smaller iPhone, that's an option too. Good luck with your decision and hopefully you're happy with whatever you choose.