There's only about 3-4 months left until the iPhone 7 is announced, so if you buy the 6s Plus now it will become outdated quickly. Wait till the 7 comes out and get it, but if it isn't that much better, get the 6s Plus.
Bought a 6s+ yesterday due to work deadline. I would've waited for the 7 if I could. Had a nice chat with an Apple Store representative. They know as much as I do (nowt) of Apple's future plans for the 7. The one thing he did say is 16MB phones are just not selling. He also noted that when they do, they tend to come back if people take pictures.
Anyone that follows the SE availability saw supplies of the 64 gb model were constrained and 16 gb widely on offer.
It would seem reasonable the 7 would have increased memory options, probably like the iPad Pro: 32, 128, 256. I emphasize this is pure speculation. it seems reasonable Apple would not want to sell a phone that has a non-optimal user experience and there may not even be an incremental build cost.