If we're going to go the Ender's Game route, then we're going to take the fight to them and launch a WMD at their home world. If we're going the Mass Effect route, we'll be welcomed into the intergalactic fraternity of intelligent alien life, and subsequently be forced to grudgingly let the captives go as part of peace negotiations. If we go with the Halo/Robotech route, we'll scavenge their technology, advance our own tech by decades, and take the fight to them (again) in an arms race over yet another alien race's even superior tech.
In answer to your scenario, though, I think we're fairly justified in killing them. The survivors would no doubt attempt to make contact with their main forces again, even if we manage to scavenge their tech and use it against them we'd still by outnumbered and outmaneuvered so soon after we liberated ourselves. Our best bet of survival afterwards would be to either mass relocate to another habitable planet, prepare countermeasures for the ensuing re-invasion to come (constructing defenses, mass militarization of the survivors, etc.), or to try to make contact with other systems they've conquered and foster the resistance even further.
And to all those in favor of adopting the survivors into our population, remember: They've had no qualms about killing us earlier. Assuming this was a first contact scenario where they opened with hostilities as soon as they landed on Earth, then we can conclude that they were either after our resources or land for expanding their population centers. Since they went straight to the mass murder and enslavement, this wouldn't be a simple regime change where citizens were left mostly alone and world governments just had to pay tribute to them. This was xenocide. Hell, probably the only reason we would even be left alive as slaves would be to act as cheap labor while they made us mine for rare minerals or as guinea pigs for genetic experiments.
