I'm comparing the two e.g. on MyFonts:
https://www.myfonts.com/fonts/mti/helvetica-now/
https://www.myfonts.com/fonts/linotype/neue-haas-grotesk/
Or specifically for the medium text:
https://www.myfonts.com/fonts/mti/helvetica-now/text-medium/
https://www.myfonts.com/fonts/linotype/neue-haas-grotesk/pro...
Aside from the fact that they're scaled slightly differently, nothing whatsoever jumps out to me as noticeably different, in any weight, except that Neue Haas Grotesk has descenders that seem unnaturally short, but I can't tell if that might be a rendering box clipping issue.
What makes Helvetica Now "such a departure" and the name Helvetica just "marketing"? Whatever difference there is seems extremely subtle to my eyes...
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.
I'm feeling like I'm in crazytown, you're saying they're "very different" and the letterforms just look... virtually identical to me. Totally same personality and all.
I'd wonder if I needed higher resolution, but you're talking about using them in apps, so that's definitely not the case.
I would add to that though, where something is being displayed matters too. If you're displaying on screen and you're not using anti-aliasing (I don't) then whoever did a better job hinting will display better on the screen.
I think one distinction is in the lowercase 'm'. Primarily around the left most stroke of the letter where the first arch of the 'm' meets. Neue Haas Grotesk's 'm' has a narrower starting stroke on the arch when it's leaving the first stroke. Whereas Helvetica has a mostly even width arch on the 'm'. This one I'm relatively uncertain on and it could be my monitor but it does seem like there's a mild difference in whatever that nook/cranny is called.
Another spot where I think the differences are apparent are with the lowercase 't'. Helvetica has an almost right angle change in direction on the inside of the base of the 't' whereas Neue Haas is smoother.
I would also look at the lowercase 'a' for differences if you really wanted to see it, Neue's is much curvier on the "belly" of the a.
Sorry if these aren't the right words, not entirely sure what to call the "parts" of a character in a font.