As a general statement, you only need the 64GB if you have a LOT of music and/or a LOT of video on your device.
That said, if you have the 64 you can always have 30GB of stuff on it, but if you have the 32 it will NOT hold 30GB of stuff. It's not upgradable! When I purchased my iPad I "wasted" my money by buying the 64GB, now it only has 11GB free.
Movies, 1 or 2 GB each, a navigation app like Navigon, over 2GB, audio podcasts not so much, video podcasts quite a lot, using the camera to record in 1080 HD, large amounts of GBs. Things do add up quickly. I ordered the 64GB 4S after my experience with the iPad.
I normally bought the biggest memory available (32gb iPhone 3GS and 4, and the 64gb iPad), but since iCloud now is out, I decided I'd just get the 16gb 4S. My thinking is this....at the most, I'm going to have this phone for 2 years (most likely just 1 year). If I need more to storage, I can buy more space on iCloud, and I'd still save money over buying the 32gb ($100 more), or the 64gb ($200 more).
I had a 16g iPhone 4 but it got full too quickly so I kept having to delete stuff which I hated , I was going to get the 32 g this time but as my 64g iPad was getting full I decided to go for the 64g might be overkill on my part but I know I will never have to delete something just to be able to update an existing app again .my g were taking up by mostly apps as I only had a few gig of music,I had loads of pics but no films on them.