I would definitely place my bet on Earth B because we are human and they are inhuman.
Assuming both parties do not have any technological and biological advantage over each other and the inhabitant of Earth B had already parked their space-readied rubbish truck next to our moon landing site and vandalized the flag already. I believe depending on the threat level posed by our neighbor we can take necessary counter-measures to defend Earth A. (wait since when did the name changed?)
Threat Levels and Countermeasures of a effectively planetary defense system:
Level 0 - Inhabitant of Earth B uses radio wave signals to spam us
Counter 0 - Reciprocate
Level 1 - Our earth bound and deep space telescopes detected UFO travelling at a fast velocity heading toward us in tight formation and spell "Earth A sucks" on our radar screen.
Counter 1 - Firstly we need to find out their intention by hailing them the Star Trek style and find out the type of propulsion mechanism their vessels used for the entire journey. (is it a one-way trip like a homing missile) Secondly we can intercept them mid-space with our trump card... the space junks good luck to them trying to stay in one piece. Thirdly we must establish a committee on Earth and reveal our top secrets weapons between countries... united we stand.
Level 2 - The UFO are within hours away from us and they are most probably launching projectiles in our direction.
Counter 2 - The committee on Earth A must be saying something like this "why aren't we the attacker and where the hell are Mars and Curiosity?". We need to consider if their projectiles are capable of penetrating the Earth A several layers of atmosphere, most crucial is what will happens if the content inside the projectile is dispersed in our atmosphere will it spread and block out the sunlight and cut off potential energy supply and turn our weather against us. We must deploy rockets to intercept them to mitigate the impact caused to Earth A.
Level 3 - Dogfight
Counter 3 - Hopefully only a insignificant amount of UFO managed to get past our defense by now and they are turning tails. Suppose in the worst case scenario these aliens penetrated our atmosphere and I believe they must be targeting our power source and supplies first. Most of us on Earth A will be located underground to seek shelter from nuclear holocaust, else we will most likely engage in a awe-inspiring dogfight ever in the history of mankind.
Level 4 - Land Invasion
Counter 4 - where are our storm troopers? our choices are scarce but not limited, our weapons should have not problem granting the alien death wish. Most probably we must drag the invasion as long as our resources last so using guerrilla tactics if we are outgunned (I remember you mentioned they are only as advanced as us therefore just deploy the seal teams or drones in covert mission to sabotage their ration and power source) I don't recommend using biological weapon even though they may be inhuman. Once they are wipe out we must send fleets to harass them on their planet and force them into negotiation, else the future of mankind and alien alike will be less promising.
Level 5 - Planetary Destruction eminent
Counter 5 - [fall silent]
Verdict: Planetary defense is absolutely fun to ensure the survival our species, so pass the bill already!