Both silk and leather are derived from animals/worms. I’d answer the question based on factors of practicality.
In my opinion, leather bags are always better considering:
- Leather is one of the most durable materials. A good leather bag can almost last you a lifetime with proper maintenance.
- Leather is resistant to daily wear and tear - silk can easily tear.
- Leather bags have an elegant appeal - both in a formal and informal environment. Silk is hardly the choice when it comes to a work bag. It’s more suited for grand occasions or evening soirees.
- Leather is easier to maintain and less risky - silk is flammable.