number 6, the -600 is very similar to the -300, but pilots still have to go thru a short training period, more like a "getting used to it" sort of thing. One big difference is the taxying of this a/c - its the longest aircraft in the world at over 75m (I believe) and can be quite different to the -300. Similar to the 777, the -300 has the same cockpit, flight characteristics, etc as the -200, but the the sheer length of it requires some getting used to during taxying and the flare during the landing, etc....

As a comparison, for a pilot to convert onto another a/c which don't have the same rating, eg 777 to 747, they have to go thru up to 2 months of training, whereas the Airbus conversions can be done in a few days. And that's Airbus' BIG selling point.