It's funny when someone makes a thread asking, specifically, why folks chose the 6S over the 6S Plus.....and 6S Plus users pop in stating or defending their choice of the plus.
6s+ here, but I tried the 6s out on launch day after almost a year with the 6+. Here's why, in order of importance to me:
Size: it took me the better part of a year to get used to my 6+. It's a big phone, and while I could use it one handed the majority of the time, it was challenging.
Performance: I was not 100% happy with how my 6+ performed and I wasn't willing to take Apple at their word that the 6s+ was faster.
Weight: we knew the new metal would make it heavier, and I was concerned about how much more. The 6+ was tiring to hold sometimes, especially lying down
Cost: I didn't want to pay $100 more to get a phone I wasn't completely happy with.
Ultimately I got the 6s+ and I enjoy it, but I can always make a good case to get the 6s. Both are outstanding phones.