RAM & Storage
One theme of the new Samsung Galaxy devices for 2018 is that the two models may not feature identical specs to each other, as the Galaxy S8 and S8+ largely did. According to leaks, the Galaxy S9+ will include a larger amount of RAM -- 6 GB -- than the Galaxy S9, which will have 4 GB of RAM. By comparison, both the Galaxy S8 and S8+ have come with 4 GB of RAM.
The same dynamic is true with storage, with the Galaxy S9 rumored to be coming with 64 GB of storage (the same as the Galaxy S8), but the Galaxy S9+ rumored to pack 128 GB of storage (twice the amount of the Galaxy S8+).