I have the problem that there's not enough space on my laptop for GTA 5, so I wonder if I should use a USB stick or an external SSD to download or save GTA?
Both are possible, but an external SSD is better because it is faster.
Then prefer a good SSD. Or if it is possible you can have the hard drive replaced in your PC.
Best internally otherwise an external m.2 ssd or an ssd because a usb stick is quite slow
Theoretically, both would work, but an SSD is much faster.
An SSD. With a USB stick you wait years 😂