ok, here's the total time I've spent for accomplishing crap:
3:30 hours of brawling (450 matches): Unllock all characters, all star mode, almost all stages, all assist trophies, most songs, and a good deal of other crap.
7 hours of Subspace Emissary: Beat it all the way through on normal. Got a fair amount of coins and stickers and trophies. Unlock boss mode
1:30 hours of events and stadium: Beat 35 events, unlock trophies and the last few stages and more.
My take on it: Although you need to complete subspace emisary to unlock some events and stages, I would still play at least 300 brawls first to unlock a TON of other stuff. Then go thru subspace. Then events.
BTW What is with that last level of subspace? RIDICULOUS!