It depends on how you use Google's services and Android.
First, Google's bet is that, with 32 GB, you'll use its Google Play Music to stream your music and Google Photos to store your pictures (which is why you get unlimited full resolution backups). Now if that's the case, then you could save some dough with the 32 GB if you're not a huge app user (and I'm talking a LOT of apps here).
If you're the kind of user who installs gigabytes worth of apps (and that a whole lot) or if, like me, you want to have all your stuff locally (I don't use Google Photos at all for security and privacy reasons, not that I don't trust Google mind you, but everything can be hacked) then get the 128 GB like I did.
Also, if you're the kind of "Storage OCD" user who gets increasingly anxious as available storage space lowers, then better safe than sorry, get the 128GB.
Now be aware that the 128GB version can be hard to find. I had to go to 3 stores before I found one.
Hope that helped!