Considering a tablet is not primarily designed towards photography, its safe to say the iPhone 6s Plus will be superior. The 6s Plus has OIS and a superior image processing algorithm as well as a superior sensor which will easily hand it the victory for image quality. On the video side, it films 4k which the tablet simply cannot do.
I'm not one to judge but taking pictures with a 12 inch tablet is not remotely ergonomic nor something I would consider doing even if the tablet took better pictures. I consider tablet cameras to be used in the rare occasion I don't have my smartphone and want to capture a picture but to each their own.