. . . what would justify 64Mb vs 32Mb?
Seriously, after the rom, apps and data you could easily be down to 20gigs. Personal videos and photos can burn that up quick, not to mention any music.
You're otherwise spending $200 (contract) or $600 (retail)...why regret the purchase down the road because you don't have enough storage over $50?
With 4k video and a 21 megapixel camera.. If you use those features alot.. They'll use up space quickly..