Just heard this from my colleagues, so not sure if this is 100% accurate:
Add comment now: instantly adds the comment to the review and notifies the reviewer that a comment has been added. So if you choose this option X times, the reviewer receives X notifications.
Submit review: You can add as many comments as you want in as many files as you want. If you choose this option at the end of your review, the reviewer will receive only 1 notification containing all of your comments.