HideoK on 28 June 2010
Haven't played Alan Wake but I hear its really short between 8-10 hours. Since its a just a single player game you should probably consider renting this...
Read Dead Redemption takes 25-30 hours to 100% the single player story and then you have the multiplayer to keep you busy for a while after that and the new free DLC "Outlaws to the End" co-op missions for up to 4 players. There's probably more DLC to come.
Edit: Just thinking about Alan Wake again and I read they have at least two 90 minute episodes of DLC coming soon. The first episode called "The Signal" releases July 27th. In that case you probably want to buy the game so you don't have to keep renting it to play the new DLC.
