if it where me, i'd use the White Labs German Ale/Koelsch yeast for a Koelsch (which I have a few times and liked) or the Wyeast Koelsch. if i was to brew an alt, i'd go with Wyeast's German Ale yeast. i've used the WL German Ale/Koelsch for an alt, and would want to try the Wyeast. again, just my preference.