This is how i made my decision.
I see the storage as 'hours of offline'.
So with 64 iPad, i can pretty much get ~40G of free space after regular documents and apps.
And for a decent quality movie, it's about 10G for 90minutes to 120minutes.
So 40G free space means 6-9 hours of offline watching.
I take a lot of long haul flights that are over 12/14 hours. So 64gb iPad is not enough to help me survive the flight. Therefore it's a no go.
However, 256 will give me more than 36 hours of offline playback, which will at least take a round-trip-trans-pacific-flight to consume. And i can delete/download/update in between flights, so 256 is enough for me.
For 512, first I don't *necessarily* need the extra 256gb. Yes i may want the extra 256 occasionally, but in that kind of rare circumstances, I'd put my money towards those wireless hard-drives (like wd, lacie, or even seagate), the same amount of money can bring me >2TB of space, which will also serve me as storage for my laptop (IF i bring one, thumbs up for iPad)