I have used them both in apps, and they both have their quirks. I excepted Helvetica Now to be slightly better/modernized version of traditional helvetica, but it is not. It feels different (not better).
Nue Hass Grotesk has its own issues. It has a very industrial feel, and make it great for single line of text as a display font. It fails if your text is over 2-3 lines (eg. news headlines). I think Bloomberg news uses it successfully, as it matches its 'industrial/business' feel, but it is not a 'warm' font.
The lesson I learned: The perfect font doesn't exist. They all have flaws.