Not sure if you wanna weigh this in as a reason to choose one or the other, but with the one-time purchase you can only stream via Plex on mobile. If you want to download the file and then watch it at a later time WITHOUT streaming you need Plex Lifetime (I made this error!)

Have you tried the DownThemAll! extension? I know it’s available for Firefox, unsure if it’s for Chromium browsers or not.

Personally use this, hasn’t let me down yet!