The A-318, 319, 320, and 321 are all the same type rating. A pilot with an A-320 type can fly any of the aircraft in this family without any differences training. I know specifically that the A-319 is 7 frames shorter than the A-320, and at least in the NWA configuration, seat 124 and 148 pax respectively (with a small first class section). I have seen A-320 configurations though that seat up to 168 pax. For all intents and purposes, these 4 aircraft are all the same with only minor differences in length. Not to mention that a transition to an A-330, A-340, or even an A-380 can be accomplished in as little as 10 days. The fleet commonality of the Airbus family greatly reduces training costs.