Well, I know someone will correct me with the right data, but for I being told the A319 has higher Nautical range than the A321 and A320, performance wise are pretty much the same.
JetBlue is introducing the A321 in other to aid some of their heavy routes and not for nautical range, example I believe that JFK
has about 10 daily schedule flights, let said that at least 8 of them are in the A320 wich means about 1200 pax. The A321 can carry that amount in 7 flights, so you can propebly make the statement that other flight that they save can be utilized to another market.
If you need capacity in selected market, then yes, the A321 will fit very well. But if you need range, A319 baby...