The PS5 will sport a 825GB SSD and the option to go big with M.2 Non-Volatile Memory Express (NVMe) SSDs. While, Xbox Series X will make use of a 1TB (NVMe) SSD and proprietary 1TB Expansion Drives (along with USB drives for older games).
Sony will have to certify each compatible M.2 NVMe SSD that will work with the PS5; Mark Cerny stated these certifications will come “a bit past” after the console’s launch. Also that, USB external hard drives will be supported to some extent.
Check out PS5's specs in comparison to Xbox Series X specs here.