I'd personally make an extra effort to get them both, really. But if you absolutely can't then I'd suggest 16GB RAM.
It's true that macOS handles RAM usage in a smart and efficient way, but you'll never get the physical memory you don't have, and you won't be able to change your mind later. External storage, on the other hand, can be added at any time, with fast external SSD drives getting cheaper and better by the day.
That said, 256GB are pretty easy to fill if you work with lots of photos and videos, and moving large files around to keep free space on the drive is a pain. If it were me, I would just bite the bullet and get both upgrades. It's not hard to justify, it's really the best choice.
It's true that macOS handles RAM usage in a smart and efficient way, but you'll never get the physical memory you don't have, and you won't be able to change your mind later. External storage, on the other hand, can be added at any time, with fast external SSD drives getting cheaper and better by the day.
That said, 256GB are pretty easy to fill if you work with lots of photos and videos, and moving large files around to keep free space on the drive is a pain. If it were me, I would just bite the bullet and get both upgrades. It's not hard to justify, it's really the best choice.
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