I actually have a kolsch I just kegged that uses the same two hops - Spalt and Hallertau. Authentic kolsch yeast is important for the style and in my opinion Wyeast 2565 is the best. On the one I just kegged, I used the Lallemand Kolsch yeast, which is a dry yeast, for the first time. It's not done carbonating so I can't comment on how it compares with Wy2565. You can get away with using US-05, it's just not an authentic kolsch yeast.