Assassin's Creed Unity hits PCs today, and both AMD and Nvidia have fresh drivers ready. Nvidia's release is perhaps the most polished; the GeForce "Game Ready" 344.65 drivers have WHQL certification, denoting Microsoft's official seal of approval. These drivers offer "the best gaming experience" for Unity, according to Nvidia, and they include the requisite SLI profile.
The release notes (PDF) don't detail any performance improvements, but they do list new SLI profiles for Heroes of the Storm and Train Simulator 2015. The existing Ryse: Son of Rome profile has also been tweaked to "improve SLI performance and reduce stuttering."
AMD's Catalyst 14.11.1 drivers are classified as a beta release. They have "peak driver performance optimization" for both Assassin's Creed Unity and Call of Duty: Advanced Warfare, but the release notes also list a handful of known issues, such as "occasional stuttering" in Unity with "specific" (but unspecified) settings. CrossFire setups can also exhibit screen tearing, texture corruption, and "negative scaling" with certain Advanced Warfare configs.
Assassin's Creed Unity employs some of Nvidia's GameWorks effects, and it looks absolutely gorgeous on the PC. Regardless of whether you have a GeForce or Radeon, you should grab the latest drivers before playing Ubisoft's new assassin adventure.