Things to consider…

1 Speed

Do you need to start your Mac up from this drive? If so, you’d be better off with an SSD. A hard drive is fine in a pinch, but slow for frequent use as a boot disk.

Will you use this drive as your working storage? On hard disks, the APFS file system can cause falling performance due to fragmentation of constantly changing files. Use an SSD instead.

Slimline cases are easily slipped into bags and pockets, but provide little protection from impact, water or dirt.

Most drives have USB Micro-B ports, so you’ll probably need the cable that comes with the drive. If that has USB-A at the other end, you may need an adaptor.