Can't see the lighter one from a different machine. The original big 9's tower over everything else. Normally if it has the same part number it is 100% interchangable, just a running change. Sometimes its just looks a bit different, sometimes its an upgrade. The heavier may not be the upgrade. On our commercial zero turn mowers the nose plate was fully welded. After a season it would crack. The upgrade was four inches less welding so it would flex instead of super stiff and crack.
Think there was a fellow on here with two or three 975's & put hours on them, who might have noticed differences. Dwayne maybe? Good Luck -Scott