The proper answer, is well, it depends.
If you are talking about an equipment that has a prober PCI bus, and a SATA/SSD disk, USB will be the bottleneck for an external disk and far slower. Possibly also more unstable too; I would try to live with the internal device and use the external disk for storage. Actually for such scenario, I would prefer an external disk shared by my AP/a NAS that I would mount via a remote drive than connecting a drive via USB.
If we are talking about an iOT device with an SD card, like a raspberry, that has not a SATA interface connected to the SoC, by all means use an external pen or hard disk. SD cards are known for being slow and unreliable.
In any case, the USB interface does not lend itself well to high volume sustained speeds, and thus video processing or other I/O very intensive operations are not advised on external USB disks over a long period of time.
Anecdotally, I have an iOT ARM device with an external SSD disk over SATA. The A21 ARM has support for a SATA interface in the SoC.