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Start with capacity and the type follows. If you need four terabytes or more for the least outlay, a mechanical portable HDD is the obvious answer and speed is secondary. If a terabyte or two is plenty and you value how quickly files move, a portable SSD is worth the premium.
The USB interface decides how much of a drive's speed you actually get. A mechanical HDD saturates a basic USB connection on its own, so a faster port buys it nothing, whereas a quick SSD is wasted behind an old port and only stretches its legs on USB 3.2 Gen 2 or better.