Well, the big hardware difference is that the iPhone 7 has the 1.8 aperture versus the 2.2 for the 6s Plus. That would mean better low light performance for the 7. In good light they are probably the same.
I have an iPhone 7 Plus and my wife has an iPhone SE (6s camera). We take lots of photos simultaneously at various events of our kids and such. There is no quality difference between my 7 and her SE in good light. Even in dim light not a huge difference. Although, the darker the setting the better my 7 Plus looks in comparison. But your 6s Plus with optical image stabilization would be even better than the SE in dim light.
Camera's are getting so good anymore the improvements are getting smaller and smaller.