According to an official document, the MK 108 weighted an hefty 58kg empty, and each round weighted about 0.47kg and ammo links around 6.8kg for 60 rds so the total weight was 93Kg.
The MG 151/20 weighted 42Kg, ammo 0.205kg each and links weighted 2.6kg so a total weight for 150 rds of : 75.35Kg
So weight difference was about 18Kg.