I think burnside was the closest rescue brigade in distance, and probably road speed aswell. Athelstone have a fair bit against them, ie long road routes and slower bendy roads.
Stirling would have indefinitely helped with possible Rope rescue if it was any steeper.
In regards to quickest road route, if a brigade has access to 100km/h roads...and makes them 10mins drive from the incident, and is physically further away. Meanwhile the other brigade has access to majority 70km/h roads, physically closer...maybe 12-15mins away. The quickest responding route is probably the best choice.