Open Question: Ipad wifi+3g what gig is rlly necessary?

What one should I get? The 16g 32g 64g what one would some one rlly want? Like i'm keeping my iPod touch for my music and I'll use my ipad for apps and movies and books. So what I wanna no how good is the 16g 32g and 64g like I have a 32g itouch and I still have lots of memory left. I wanna no how far each will get you memory wise.