As a rule of thumb for myself, I always go with the highest amount possible. This being the 128gb. I like the peace of mind that it gives me !
I take a lot of photos and videos, so knowing that I could record a video for quite sometime before running out of memory, is a very comforting feeling......
Also, if you are willing to go with the 6s plus, I would highly recommend spending the additional $100 for the added gb. Just my $0.02