Ever find yourself, or see others, saying the alphabet to determine which letter comes next? Typically not the same with numbers.
The decimal system is instantly understandable. Three numbers mean it is a third-level section.
The traditional system is slightly harder with few readers instantly knowing “a” is a fourth-level heading – 1.A.1.a.
If given a choice, select the decimal system over the traditional system to reduce the cognitive load on evaluators having to rebuild references delivered through the traditional system.