MTGA is a more visually appealing version, MTGO has more for the older players(such as older formats)
MTGA also is more beginner friendly, very easy for a person with little to no magic experience to jump in and learn the game quickly.
To answer your question about the Sealed Stuff, pre-release events in paper have codes for MTG Arena, not for packs but for 1 time entree into a sealed event on MTGA.
This post was edited by Keiga on Dec 7 2018 01:03am