Owning both and having played both from start to finish. Plus having had adequate time to ruminate on the matter. I can see your dilema. That said the question isn't so much a question of quality as it is a question of quantity.
Alan Wake is a good game, but you are looking at a fifteen hour play through. Further more the game is fairly linear. So if you are really an achievement whore then you are looking at forty or fifty hours max before you exhaust the game totally. That said you will probably find this game at a good value in a few months after the next two expansions have come out for the game. Either used or on sale.
Red Dead Redemption is a completely different beast. Your looking at a twenty five hour main story line, but it will probably take you longer doing the side quests. Your probably looking at forty to fifty hours straight up, and a hundred or so hours if your an achievement whore. Further more the game has solid mini games included, and offers online play. That can get you another dozen hours or so easily enough. That said this game isn't going into a bargain bin any time soon. That said it isn't just GTA on horses. Sure the old GTA dynamic is there, but on the other side of the equation there is robust role playing aspect to the game. You can spend hours just hunting, foraging, and doing randomly generated missions. So you don't necessarily have to play it GTA style.
Alan Wake is a great game, but its design limits its value. While Red Dead is well ahead of the value curve. You will probably do better by paying sixty dollars for a game that will easily give you a hundred hours. Then a game that will only give you fifty if you really drag things out. Good for you that you plan to get both regardless. Both are really good games.
