If Barca's playing with Scrubrique tactics where the central midfielders are wingbacks => Mascherano. He is quicker and more aggressive than Busquets to sweep up behind a non-existent midfield.
If Barca's playing with recent GOATrique tactics where the central midfielders are actually central midfielders => Busquets. He's technically superior, better under pressure, better in small spaces, a better linkup player and a better passer between the lines.
Simple, really. If Barca wants to fight the symptoms of shitty tactics instead of fixing the root problem then Mascherano. If Barca is playing how Barca should be playing it's Busquets. I'm not a believer that players like Busquets and Iniesta suddenly turned to shit with 26 and 30 respectively. Might have lost some spark from their heydays but I'd always question the system first when Busquets AND Iniesta suddenly look ordinary.