For me if I had the money and was choosing one, probably 32 GBs. 16 GBs seems to be so small you would have to treat it really as a toy/extra supplemental device where as 32 gives you more space to be at least more functionality. It's still not ideal (I already have my 32GB iphone almost filled) but it would make it workable as more than just a supplemental extra.
If I were planning on using it heavily or as my main device (or even as my main portable device), it would be worth it for the 64 GB.
So it really depends on how I saw myself using it. If it was just a neat toy that I wanted to be able to mess with, I could see myself going with 16 gbs. If I saw that it had some functionality I'd see as useful but I'd still mostly be relying on my laptop, probably 32. If I saw it as either something I'd be using constantly, or replacing my laptop when I go on trips (I really don't see that for me though, the laptop to me is worth the extra weight to take with me), I'd go with 64 GBs. But I would probably have had to had one before and this be an upgrade and seen my use of it to convince me to pay 700 dollars for a tablet honestly. Even 600 seems a bit much, it's just that 16 gbs seems pretty useless so they could probably convince me to at least pay that.