There are 2 options for brakes, the 70 mm and the 90 mm.
If we put a 70 mm drum brake and the brake plate, then we compare this with the 90mm, we can see a difference of 460 grams. This is the disadvantage of the 90mm drum brakes, they are heavier.
The advantages of the 90 mm drum brakes are the following:
1) They can better absorb heat when you have longer downhills to do. We just saw on the scale that the 70 mm drums are lighter. The 90 mm have more material to heat and have more surface to get rid of the head. This mean you can brake longer without having the danger of overheating.
2) Because the 90 mm drum brakes can get better rid of the heat the have also less fast oval
Conclusion: You live in a flat area and occasional you see some hills then a 70 drum brake would be enough but you have faster change that the drum brakes get faster oval.
You life in a area where you go often up an down some hills mountains 90 mm is the best choose in this case