Just remember that everybody's usage case is different. Just because you didn't fill your storage space doesn't mean that someone else wouldn't. Not busting on you. I just want to make sure that everyone realizes that there is no definitive answer.
Everyone's usage is different, but people do generally fall into one of two types.
First is the type who, once they have enough storage for the OS, regularly used apps, and perhaps sufficient media/games to provide for a couple of days consumption, don't normally fill the rest of it just because the space is there. The extra room going to a larger capacity model above the one that does suit their needs is seen as, and would be, purely a luxury, not a necessity.
Second is the sort who, after having enough storage for the OS, regularly used apps, and perhaps sufficient media/games to provide for a couple of days consumption, will throw on media/games they might like to listen/view/play at some point, and will migrate towards the largest available version out of concern the 32GB would not be enough. The reality is, though, that for most of them, going to the larger capacity will only provide a bigger closet that they will be as likely to fill and find themselves out of room.
Now I don't want it to appear I'm knocking the latter - I tend to be of that type myself. But recognizing that about myself, I opted for the 32GB, knowing I could manage with its storage, because I could actually lay hands on one much more easily and sooner than on the 64GB model.
I suspect nobody who buys a 64GB model over the 32GB will regret it - at only $50 for the upgrade and offering more than twice the available storage, it's not a bad deal at all for the convenience it provides. But by the same token, I suspect that many who, like me, purchased the 32GB simply based on the elusiveness of the 64GB, are not feeling as limited by it as we might have initially feared.