I think at the end of the day you have the make the determination for yourself, I've tried both extensively and ended up going with the 6S. It's hard now to find a compact flagship device and the 6S gives me that.
I really like the Plus model, it's very fun to use and play with in the house or when you're stationary, but when you're on the move and actually mobile, it starts to get cumbersome. Battery life is amazing on the Plus model but the regular 6 easily gets me through the day with my usage.
I would say if you're looking for a truly all in one device where you don't need a tablet or use a computer very much, and take lots of pictures and do a lot of media consumption on your smartphone then thats where the Plus is good at, otherwise if you're just looking for a good smartphone then I would stick with the regular 6S, just my two cents.
I've used almost every iPhone since it first released and it's always been compact of course, so thats why I still stick with the smaller model, when I'm home I'm either using my computer or my tablet, and my phone just lays on my desk most of the time, when I'm out and about however, I want something more compact.