I've really enjoyed the views of the Tarantula, Carina and Orion nebulae through the NPB, most recently in my C11 they were unreal - I'd never seen anything like it from one of my own scopes. Mental's 17.5" is a beast
So far, I've found I've preferred the views with the NPB over the OIII on the above, but I've only had the OIII for a couple of sessions so it's early days. The NPB just looked more wispy and extended. No doubt my choice of subjects is a factor too!