The 32 seems to be the most popular. That's the one that seems to be sold out everywhere. When I bought mine they prefaced it by saying they were all out of 32, but had one 16 and five 64 left.
I think 16 is more than enough space. I hardly ever come close to having more than that on any portable device. And my primary use for the iPad is not to store all my videos and music files.
That's what the iPod and iMac are for.
