Donkey Kong Country Returns:
-seems like the safer choice with the tried and true DKC gameplay
-the gameplay is more fair to the second player (seeing how Kirby games don't have the first and second player work on the same mechanics, especially when it comes to the way to summon the second player, though I'm still aware that Donkey and Diddy Kong will have different playstyles)
-will probably have the better balance of enjoyment and difficulty
Kirby's Epic Yarn:
-I like the unique textile graphics (I was also a big fan of Kirby's Dream Land 3's hand drawn graphics)
-will probably have more replay value with all the secrets that Kirby games have (I find it more preferrable than finding all the Bonus levels in DKC games)
-the new yarn mechanics look fun
I'm kind of leaning towards Donkey Kong Country Returns. Though that's partly because I thought Nintendo gave up on Donkey Kong games for good, since they sold Rare to Microsoft. Seeing the Donkey Kong series still alive is making me show more a bit more regard to it than Kirby.