Should I upgrade to the iPhone 6s plus or iPhone 7?

I'm almost finished my two year contract for my iPhone 6 and will upgrade next month but i can't decide if i should go for the 6s plus or the 7. The 6s plus seems a little big for me but everyone says you get used to it really quickly and it looks like the performance of the 7 isn't that different. The battery life and the fact that there's a HEADPHONE JACK (still so mad about the 7 not having one.) is a big plus for the 6s plus. But I'm worried that it won't perform that well in two years time. I really am not sure which to go with any opinions would be really helpful.

There's a big performance difference between the 6s and the 7, anyone saying otherwise clearly hasn't tried both devices. As for the headphone jack, the iPhone 7 comes with a pair of headphones that work, it also comes with the adapter so you can use normal headphones as well