Not as good as the Monroe and Drummond duo.
Obvious. Captain.
Underlying point (if not obvious): Monroe and Drummond aren't exactly lighting the world on fire as a duo... so it should temper your enthusiasm for the Monroe and Sully duo. It clearly wouldn't be as good as what the Pistons have going for them right now as-is.
Sullinger isn't a lesser version of Drummond, he's a different type of player. Better scorer but not the same rebounder. Another way of looking at it is to compare Monroe to Bass and consider the improvement to our lineup.
The current duo they have is a lot stronger than a Monroe/Sully duo
Drummond anchors their defense and is definitely a better rebounder.
However, if you're talking about our side, Greg Monroe is definitely an upgrade over Bass, so yes, in that sense, GM/JS is a better duo than BB/JS