One thing I really like with the iPhone 6s Plus is the bigger display. The better screen real estate.
It's very useful when browsing the web using Safari for example. You get more visible content on the screen at the same time. It's also very useful in apps that support iOS fully so that's the text reformats too fit the larger screen better. It makes the text bigger and easier to read. For example the excellent email app Spark does this. It's a pleasure to use these apps.
Unfortunately is Google terrible at developing iOS apps, so none of their apps looks good on the larger display that the iPhone 6s Plus has. They seriously need to get rid of the terrible way they develop apps and start from scratch by supporting Apple's UI guidelines 100%.
Also when blogging in the excellent app Editorial is the bigger display very useful thanks to the better screen real estate. You can see a lot more of the text you writing. I really like to write and blog on the iPhone 6s Plus. It's almost perfect for that.
And of course if you need to watch videos on your iPhone, the larger display on the iPhone 6s Plus is great for that.
Also gaming is of course better on the Plus version of the iPhone, but personally I think the iPhone 6s Plus is too heavy for that. If you game on the iPad you can always use your knee or stomach as a stand, but not as easily with the iPhone 6s Plus. So you hold it in your hands more while gaming and it strains your hands and arms.
The larger display doesn't helps much for video editing though, it's still too small to be good for that. It is manageable though and I use it's for all my YouTube videos now, but I must admit that it was much easier to do video editing on my iPad Air 2.
Same thing with audio editing. still too small to be good for that but I do it on the iPhone 6s Plus anyways, even if it's much easier to do on the iPad.
But all reading is much better on the bigger Plus version of the iPhone. I read a lot of ebooks on my iPhone and the iPhone 6s Plus makes that experience even better with the larger display. It's much bigger but still not too big to be easily used while laying on my side in bed reading. I can still hold it very comfortably.
Same thing with reading news in the apps I use for that. For example Pocket, Tweetbot and Flipboard. The user experience is excellent for that with the iPhone 6s Plus.
So the larger display on the iPhone 6s Plus gets a huge plus from me.