The Borg seem to have prioritized--in the region of the Azure Nebula came the Federation first, then their Klingon allies, then the Romulans--but they weren't selective. Their end goal seems to have been to destroy all the independent civilizations, the ones to be included in the Typhon Pact, too, within reach of the Azure Nebula.
That actually prompts an interesting question: When would the Borg have stopped? They apparently weren't bother to assimilate much of anything, be it people, ships, or planets, even when they were winning. They were just destroying everything in their path as they come across it, so they weren't discriminating and just attacking the Federation, but they had to stop at some point. Would they have targeted anyone that had that the Federation knew about and then just stopped and resumed their more leisurely pace of galactic assimilation once the last person who'd ever heard of Earth had been killed?