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!)
Heck yeah, have about 5 or 6 extensions I use on the desktop version I wanna use on the mobile version!