I think the size you buy depends where you are going to use it...
For example, with apps like Air Video and also the latest update to iTunes Home Sharing, every iPad has just as much storage as your main computer as long as you are in the house

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If you are going to spend a lot of time away from home then ask yourself, would I have my laptop too? If the honest answer to that is 'yes, of course!' then you still find yourself with an iPad with plenty of storage (when you have your laptop set up).
This leaves you with one situation where memory will be useful - on planes, trains or places where you don't have your own Home Share network set up. In these situations you THEN need to ask yourself, how indecisive am I ? If you are very indecisive and as such need a lot of choice infront of you then get the 64Gb, if you know your own tastes fairly well you can sync before your likely to travel and put a smaller choice of what you are likely to watch on the iPad before heading out.
Obviously there are other motivations for buying larger too, such as storing all your pictures or even portfolios (if you are using it for work). But if that is the case you will most likely not be worried about paying the extra £/$100.
I currently have my iPhone 4 loaded with 20Gb of music, which is about 80% of my library, which is based on a smart playlist, if I hear a song I'm not keen on when on the train, I drop it below 3 stars and it is kicked off my phone automatically next time I sync, if I hear a song I like when at home I just make sure it's at least 3 stars and it is popped back onto my iPod, upto a limit of 20Gb.... I then stick a few films and tv shows on it to fill up the remainder of the space, once I've watched them I re-sync.
A quick thought for all those people worried about syncing every three or four days to keep a fresh set of movies on your computer - less than ten years ago we were all overjoyed when mp3 players negated the need to swap a CD over every time we wanted to listen to a different artist... Syncing once a week in comparison doesn't really seem too bad now does it?
To summarise, I will probably be getting a 32Gb as it's a good balance of needs vs costs. Buy what you can justify but consider first the ways in which you will use it. If you use it at home / in your own network a lot - get the smallest you can cope with getting. If not, get the largest you can afford

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