I'm summary, I recommend  the S6 for its hardware and 6S for its software. 6S is easy to learn for all level of users, while S6 can be a lil challenging. In terms of functionality, S6 flexibility allow easier to get task done compared with 6S. At the end, its up to the user's personal preference and usage.
If it still does not make sense to you, just imagine cars, which performs better, automatic or manual?

just my personal opinion and may differ from others.

You should really try the latest TW hands on yourself and see how it goes. Touchwiz has come a long way since the last time you tried one.

Hmmm..
Weird that you experience this. But I find IOS is doesnt have the advantage of being smoother and more fluid than the latest android systems and I hsve both the latest android and iphones at my hands. The Iphone 6S/6s+ is much smoother compared to the 6/6+ though with its 2gb ram.