Super simplified short version:
Damage is (WeaponAssault+EidolonCharacterAttack)*(EidolonEle mentalAttack)
As such, when you have 100% more damage from weapon skills, adding 40% Character Attack gives you 20% more damage. Adding 40% Elemental gives you 40% more damage.
Unless you're really desperate for damage or your weapon grid is utterly crazy, it is generally better to go for 1x elemental 1x HP than 2x elemental though.The encyclopedia in Sanahtlig's toolbox gives you a quick peek on all the upcoming SSRs, including P2W Eidolons (who are an insane boost to your damage). Fire and Thunder are soon.