Are you flying on a DOmestic sector or a European Sector?

If it is domestic then you want to be in a seat forward of about row 14 which is where the seat pitch changes from 34in to 31in. Otherwise there is little to say about any seat in the 3x3 Domestic Config.

If you are travelling in Europe then the best seats are the A and C seat at the front of the economy cabin - because they leave these two seats in the wider Club Europe configuration. The size of the CE cabin varies on each flight according to demand - they just move the curtain and convert the seats to 2x3.

I think row 9 is the exit row on an A319 and it does have more legroom - about 37in so worth getting if it is part of the Y cabin on that flight.