Sorry for the necro bump, but after stumbling across this thread, I decided to do a Scorsese and Spielberg rewatch.
It's a toughie. Scorsese is the better filmmaker if you're looking for artistic merit, but Spielberg pretty much trademarked the magic of cinema. Scorsese has some impressive highs with Mean Streets, Raging Bull, and Goodfellas.
Both directors have underrated movies, with Scorsese there's After Hours, the King of Comedy, and The Color Money. Spielberg's underrated works are Empire of the Sun, A.I, and War of the Worlds.
Spielberg and Scorsese both have duds, the former being 1941 and The Lost World: Jurassic Park, and the latter being Boxcar Bertha and New York, New York (what a slog).
Ultimately, I think the post-2000 period pushes Spielberg ahead for me. Scorsese has gotten into a mode where he makes these fat overlong movies that have some brilliant stretches, but never fully satisfy (Silence and Wolf of Wall Street come to mind). I don't think any of them are on the level of Munich or Minority Report, as fun as Catch Me If You Can, as interesting as A.I. Artificial Intelligence. Even Spielberg's semi-biopics are better, I'll watch Lincoln over The Aviator again any day of the week. The only Scorsese movie I love from this century is Hugo, and maybe The Departed.
Also, unpopular opinion alert, while Taxi Driver is a good film it's the most overrated Scorsese film of all-time. All praise to Marty and De Niro for trying their best to overcome Schrader's obnoxious screenplay that's far too interested in siding with Bickle. There's nothing here about 70s pre-Giuliani New York or the codes and conducts of violent masculinity that Mean Streets didn't already do better. And its ending is awful.
And Spielberg has Jaws, which is the greatest movie of all-time. So ya know, there's that too.
I also admit I'm a bit bias because I love Spielberg's schmaltz, and I'm willing to rewatch his movies more often than Scorsese.