Export video or unmodified original video?

Have you edited the video? If so:

Unmodified Original is exactly that - the video as it was imported to Photos.

If you have edited the video then the other export options yields the edited version and allows you to say the quality.

If you haven't edited the video you can still export at a different quality.

So, at heart, if you want the video as imported, use Export Unmodified Original. If you want the edited version (should there be edits) and/or if you want to change the quality of the video, then use the other option.

Also, the "edited" version includes comments you may have made in the Captions section, Keywords you might have added, a Title, changed dates or locations, and anything you've done since you took the video. As Yer_Man said, "Unmodified Original" is exactly that," and has none of those things.