I currently have a 32GB 3rd Gen iPod Touch and I personally think 32GB is a nice comfortable size. I have tons of music so I would never be able to fit it all on there (not even if I had a 64GB) and my music mood usually changes so every few weeks or once a month I will rotate the type of music I have on my iPod Touch. It doesn't take long to transfer stuff so I do this and fill up at least half then leave the rest for games, other apps, photos, and videos. I never have much of anything else to actually fill the 32GB. I will put movies on there for my daughter but they will be deleted once they've been viewed.
Apps will start taking up a lot of space quickly but it isn't like you will actually have hundreds of stuff on there that you play daily. Just make sure you delete games you no longer play. Don't forget you can redownload any app using the same account without having to pay again so don't be worried to delete something unless it is a game you play daily.
However, on the other hand. If you have the extra cash and don't think it is much of a difference or if you don't have anything else you would spend it on go ahead. The 64GB is a good deal I just personally think I could spend the extra $100 or so on other items instead of storage on an iPod Touch I probably won't even fill. That $100 saved would buy a ton of games and apps.