I don't see myself realistically keeping a phone more than 2 years (as a general rule it's been 1, but I might just keep the N6 a 2nd), so only want to make sure I'm getting sufficient storage to keep me happy for that amount of time. I've been fine with the 32GB N6, and only had one time I actually found myself with a memory shortage - and that was on a vacation where I'd preloaded 3 HD movies and was shooting a lot of pics and vids, but simple to resolve as I just deleted the movie I'd watched on my flight out.
For anyone who's debating between 32gb/64gb/etc, value is obviously a factor or they'd just go with the largest. That said, get the size that would easily handle your current usage plus a reasonable expected increase over the term you expect to keep the phone. Your storage needs won't change much unless you're going to be changing the way you use the phone dramatically (note if you plan on doing a lot of 4k video.... get a bigger size. Though I still mock a friend of mine who did that, shoots all his video in 4k, but doesn't have any display devices capable of handling it.
If I move to the N6p, I'd probably go with the 64gb just to have a little extra legroom, but would save the additional $50 vs the 128GB as I'd never likely miss it in the next 1-2 years, and that $50 could be better spent towards accessories (I'm lookin' at you, USB-C!) or saved as an advance '$50 discount' on upgrading next year.