I've had a 32GB w/ both the original and 2. Due to a major screw up on my part I erased my last backup when I sold my 2 and had to start from scratch this time. I bumped up to a 64 b/c my 2 was using 28GB and I figured if I took any videos or stuff I'd easily be stuck.
A funny thing happened when I started over though. I picked out 10 books, 128 apps, and 2 videos. I don't put music on my iPad b/c I have it on my iPhone for on-the-go, and my Macs @ home. My usage came to 11GB. I was stunned it was so little because I have some big apps like Madden which are nearly 1GB by itself. So it's clear to me I would have been just as well off with a 32GB if I just paid more attention to apps I never, ever, ever use but neglect to remove.
I think the best way to figure out what you'll be putting on it and then add 8GB for future adds. There is a legit use scenario for every model, unlike some products which have a low priced one that is useful to no one but gets people in the door.