I'm upgrading for the Plus (have the standard 6 now), Camera, more RAM, in that order. I do think 3D touch will be appreciated more once we start using it.
I'm willing to bet when people start using 3D touch they'll wonder how they managed without it. I'm ridiculously excited about 3D Touch, 2GB RAM (PLEASE!), and faster processor speed. The rest holds no interest for me. The 6 Plus has just gotten much, much slower in iOS 9, so I'm happy to upgrade (Thanks, Apple!).
Check out this what-appears-to-be real-time video 3D Touch from Apple. So much win..
Watch the video