i based my purchase on having owned an iphone since it's inception. ive always opted for the largest capacity available and i currently have a 32GB iPhone 4.
i finally bought an ipad, going for the 16GB wifi version based on my iphone usage... after 7 months with the iphone, out of 29GB free, i've consumed 11GB of that, and 8GB of that is music. i had decided before purchase that the ipad wouldnt have any music on it (until they release a 128GB version, my 70+GB collection will stay on the macbook air or in the cloud with iCloud, if the rumors pan out). so that left me with 3GB of space being consumed on the ipad if i were to have the equivalent apps on it.
i transfer movies as i need them and rarely keep large media on the device once ive watched them, and i stream most of it anyways.
plus i never leave home without the iphone... tethering for iphone on at&t doubles my allotment to 4GB for only $20 a month, so $5 cheaper than to maintain a separate account on the ipad. plus it's shared so i can use as much as i want on either device. tethering using bluetooth also tends to use less power in my case and unlike wifi, i can connect the ipad without having to manipulate both devices.
it was after all of those considerations that convinced me that the entry-level ipad was all i needed, but i had budgeted enough to cover the 64GB 3G version just in case i got last minute penis envy.