I'd assume that Susanoo has much, much easier time actually reaching that cap though. I doubt Ares will reach the cap without double Belial, whileas even F2P Susanoo players can probably reach it thanks to the Ability+ that Awakened Susanoo has. You'll probably need to wait for many months to get the Darkness P2W Eidolon as your secondary though.
If you look at their first skills they may seem similar, but I speculate that that is far from the truth. I'd say that the actual value of Susanoo's Awakening is in the Ability+ that she gains (which is only good for multi-hitters like her and Ares), whileas for Ares the value comes the longer self-immolation combined with the improved combo+ passive.