Civilizations one: the Pantheon Alliance.
A fantasy civilization who's technology level is about the same as the Roman Empire or early stages of the Renaissance.
Elementals: control the elements (earth, fire, water & lightning)
Telepaths control animals, plants and (the stronger ones) humans. Most can only control a dozen of so creatures (or humans) at once, but those that are consider gods and control entire cities.
Magnetic field manipulator: control metal using magnetic fields.
Teleporters: able to teleport across the Universe. Some are also able to open portals and leave them open, allowing people to cross between worlds without a teleporter physically present.
Gods: each planet colonized has Pantheon of 12 and only 12 gods. The gods are extremely powerful magic users (capable of destroying cities in minutes and entire continents in a matter of days). Whenever one of the twelve dies, their power is transferred to another magic user, who then joins the Pantheon (Gods are not immortal. They age, so they must be replaced every generation).
Each Pantheon has two Gods who serve as representatives. One is in the Alliance, which is a legislative body made up of Gods making the laws for the entire Alliance. The other is in the Teleportation Corporation, which handles all travel between planets.
Magic users (not counting the Gods) make up about a fourth of the population on most colonized planets.
Second civilization: the human Union.
Planet-destroying weapons (rarely used, since habitable planets are rare and extremely valuable)
Light-speed-traveling ships: they take several decades to reach their destination but time slows down for the passengers as they move through space close to the speed of light. Making a trip that last decades feel like a matter of weeks.
Interplanetary communication: a Network that allows sending of electronic signals through space 100 times faster than the speed of light.
Other than this, their technology is pretty much the same as ours (this includes nukes and bio-weapons).
If a conflict arises between these two space-faring civilizations over habitable planets, which one would have the advantage and why?
More details: this particular conflict takes place when both sides lay claim to a habitable planet. The planet is 22 light years away from the space occupied by the human Union.
Both sides are aware that the other has laid claim to the planet, so both sides will arrive prepared for conflict.
The pantheon Alliance will, however, arrive 22 years before the human Union.
Neither side wants to destroy the planet as it is an important resource, and habitable planets are not abandoned in this universe. Because of this, either side will not use weapons that they think might be dangerous to the planet's environment.
But any weapon that can kill off the enemy without disrupting the planet's environment is acceptable to use.
each party controls about a dozen or so planets each once with various degrees of habitability.