You can't go wrong with either, if they are the sort of game you're into. For me Demon's Souls appeals to me more in terms of type of game and general atmosphere. But you don't play Demon's Souls for "fun". You play it for the challenge, and fun is a by-product. Dunno whether Alan Wake is more on the fun side or challenge side.
@ "but for some reason i just dont like demon's souls atmosphere. it feels kind of grey" It is, after all, a game based in a world that has been overrun by a demonic "colourless fog"? I think there'd be a real wrongness about the game if it didn't exude a depressing greyness.
If you don't like the atmosphere Demon's Souls intentionally and effectively creates then it's not a game for you and the answer to your question is a no brainer: get Alan Wake.
Demon's Souls is GOTY-worthy, and it deservingly won a few GOTY's. I think Alan Wake will also be at least nominated for overall GOTY on some sites / magazines, and like Demon's Souls it may even win the odd one.
I think in terms of generation defining games Demon's Souls will stand above Alan Wake. But Alan Wake will be there among the list of games that stand out from the crowd when this generation draws to a close.
“The fundamental cause of the trouble is that in the modern world the stupid are cocksure while the intelligent are full of doubt.” - Bertrand Russell
"When the power of love overcomes the love of power, the world will know peace."
Jimi Hendrix
