Before jaxapollo goes nuts:
6S Plus Positives
- Larger screen size
- Higher PPI
- OIS in both photos and videos
- Good deal more battery life
- Landscape support for a few things
6S Plus Negatives
- Weaker ergonomics/harder to use one handed
- Weight
- Price
- 0.5% weaker performance, because that's a really big deal and Huelen will point it out.
tldr;
it's personal preference. If you'd like a bigger screen size and better battery life, get the Plus. If you'd prefer smaller, cheaper and arguably more comfortable get the regular size.