If you look at the A318 thread that's around somewhere on here the general feeling is that LCO's are currently using the A.320 as the baseline and most expect to convert to the A321.
That's the reason Airbus bought the programme from Bombardier - its fits in under their current main offerings. An LCO needs a lot of seats (relatively speaking) because of the cost structure. Use turboprops for the real low cost/short range stuff and jets with over 100 seats for the mass market. Certainly in Europe & ME that'll be the pattern