I don't see much reason to go for the A318. It has better fuel economy, but the A319 can carry 6 more people. They both require the same amount of crew.
Soooo...... which to choose ?
A319
A318 is good when you have a large fleet of A32X family and enough money in ur acc that you won't notice much even if you do some loses :)
Cheers
VJC
A318 definitely.
It's cheaper to buy, costs less for maintenance, less for fuel, and carries a mere 6 pax less than the A319.
Frequency = win
I'd chose the 320 or 321 unless you need the extra range provided by the 319. The 318 should be avoided.