I have a 6+ 64gb, and I only preordered the 6S (no plus) because I want to downsize. Otherwise, I would have just kept my 6+, and wait for 7+. The "upgrade" from 6+ to 6S+ is not that big deal to me. I never felt any lag or issue on my 6+ that other reported.
That 6s is going to be wickedly fast compared to your 6+. Aside from the CPU/GPU upgrades, rendering everything at a lower resolution and not having to constantly downsample will make a world of difference.