Kölsch has a lot of variations. Reißdorf, Peters and Mühlen are more on the sweeter side, whereas Gaffel, Gilden, Sion are more on the stronger side (taste-wise). I mean, there are so much different Kölsch-brands, and they do taste differently. I mean, yes, you can actually distinguish a Kölsch from an Alt, but I personally can distinguish the different brands from each other. Plus, it is a big difference, if you drink from tap or "Pittermännche" (which is a 5 liter ceg), or from a bottle. Really a big difference! You need to drink it from the original 0.2 l "Stangen", the small glasses. This is the real thing.
PS: My fave Kölsch is Peters. Sold in small 0.33 l bottles, really good stuff. But here is a brewery in Cologne, like a micro-brewery, and they do Kölsch the really traditional style, plus the kitchen is original "rheinisch", lots of very delicious stuff. If you ever visit Cologne, this is a must see:
http://www.paeffgen-koelsch.de/
It is right in the old red light district, easy access.