As I know it Aedui and Arverni are supposed to be almost equally matched. If anything, the only difference between the two is that their strategic situation, which is often more pear-shaped for the Aedui, while the Arverni get a nice location. That said, in AI battles the Aedui wins almost every time, only once in my 8 EB campaigns did I see the Arverni dominating Gaul. The Arverni often just sit tight in their provinces and churn out fullstacks.