As we come up on the event, it looks like chances are the 6 will indeed be staying but the 6 Plus will be leaving. I have a hard time thinking that Apple would want to sell a phone that has a much bigger screen and higher res display for the same price as the 4.7' 6S. The Plus model always felt like the "one-time alteration", and I expect Apple to try and push the 6S Plus by getting the 6 Plus out of the way. And I know this is not a big deal, but I think Bendgate actually had some impact on Apple (which is probably why their going to 7000 series), so if I were Apple, I would be very cautious about selling a product whose design problem has already been fixed in the newer one. If for whatever reason Apple decides to keep the 6 Plus, shame on them, they'll be losing a lot of potential 6S buyers by selling a one-year old model. It's all about that revenue...
I expect the new line up to be (in order):
iPhone 6S Plus-16, 64, 128 GB
iPhone 6S-16, 64, 128 GB
iPhone 6-16 GB (yuck 16 GB...)
iPhone 5S-8 GB (even more yuck, 8 GB...........)