It completely depends on your preference. Both are promising to have great stories, and each has multiplayer. If you're more into FPS and large multiplayer battles, go with Battlefield 3. If you'd rather play a third person game with stealth assassinations, and a tad slower multiplayer experience, go with Assassins Creed. Once again it all boils down to which one you'd rather play. My advice is to research both games and then decide which one you think you'd enjoy more.
It takes a few days depending on where you're located in relation to Gamefly's shipping locations. It takes me 4 to 5 days to send and receive games. If it goes past a week and you still don't have yours, email the support team. :)