I have used both, PLEX for about 1 week, and Emby since it was known as MediaBrowser (MB). Emby has been extremely simple, very configurable, and has a lot of clients. I have a lifetime Premiere account so I am clearly all in on their product.
I would love to see this nag screen as well. As I refer Emby to friends, some may not go for the Premium account. Is this on the server side, or the client side? Client side I could understand because they want you to get the Premiere account and it really is worth it for all the other server-side features you get.