Just Google Geekbench for the 6s and 6s Plus and you will see the 6s has 1Gb of ram and the 6s Plus has 2Gb of ram. It was leaked back in August. So if you want the 2Gb you need to get the Plus.
What does logic tell everyone? Think about it. All the specs are higher, CPU is faster, 4k video, so on, so forth. Why would they only have 1 gig of ram at this point? It won't.
I don't think there's any way to know until Anandtech runs some benchmarks. Maybe just maybe one of the early access developers will leak that spec, or one of the hands-on reviewers will comment on its ability to retain many app states and web pages at once (it would have been easy for them to check).
Though my strong leaning is 2 GB. Reasons:
It's the A9 chipset
3D Touch is claimed to be quick, and it's opening previews of items in other apps. That's heavier multitasking than before and with the iPhone 6 models already pushing the limits of the 1 GB of RAM, I've GOT to think the 6S models have 2 GB just to make 3D Touch work quick and smoothly, which is the whole idea of that feature.