The best course of action would be to aid the peace-lovers and, should we triumph, destroy their naive asses immediately afterward. It is quite clear that they have enemies, and given their mass powers of technology they are liable to be attacked. Siding with them would get us caught in the crossfire, and who knows if there's a race of ultra-advanced war-loving aliens trying to wipe out those who we just declared love for. Oops.
And yes, I know you said that we can't kill both, but your depiction of humanity is generalized and stereotypical. I think that galactic security overshadows being power-hungry and money-grubbing and having penises.
