If you get 512GB now, there is no guarantee that you can upgrade in a year or two. Remember that Apple makes proprietary PCIe SSD. As of right now, the only third party is OWC which had compatible SSDs for the 2012 RMBP and they don't even get anywhere close to the 768GB SSD that is out. For all you know, an even faster standard could come out in a year or two and you won't be able to necessarily "upgrade" unless you find it on ebay, which may or may not be safe.
Let us know what you decide to go with!