You appear to be mixing up Memory and Storage Capacity. The amount of memory (speicifically the RAM) is the same regardless of the storage capacity of the model you choose (or is when comparing different models from the same generation). This memory, (which is something like 512Mb or 1Gb, I forget which but it's actually not that important) is important when it comes to performance, but not in the same way as for PCs, and because there is nothing you can do about it, they all have the same, it's not worth including in any purchasing decisions.
In terms of how much Storage Capacity you might need (16/32Gb) it's a matter of what you will use it for, rather than anything else. Consider that the same OS is supported on an 8Gb iPhone 4, the base OS is less than 1Gb total and a ralatively minor part of your total storage. Obviously the percentage of your storage that the OS takes up is doubled each time you half the capacity options, but it would only ever be an issue for you in terms of upgrades if you completely filled the iPad with apps and data, and only you can know what usage you will do as to if that is likely. Games and video editing/lots of photos/music etc will eat up space, other apps less so. I have a 16Gb iPad adn was initially concerned it would not be large enough given the size of new retina apps, but that is unfounded. Having said that, I have no video or music on it, I stream all that.
Future OS updates will be carried out (in all liklihood, if the past is any indicator of the future) on a generational basis, not on specific models within that generation. So your your original iPhone 4 supports the latest iOS release, you can be sure that it is all models of that phone that have the support, not just the ones with more storage.