If you can afford it and won't miss the extra $100, get the 128. I have 38 GB used up on my 128GB iPhone 6 and it doesn't even phase me at all. Having nearly infinite space that you simply don't have to worry about is wonderful.
When HERE maps comes back to iOS sometime next year, I plan on downloading maps for the entire United States because, again, I don't care if it takes up to 5GB since I have so much space to work with.
Take a ton of pictures and video? No problem, because you have TONS of space.