What is happening in the links at the moment may be very different in a few years time. A quick command on the 1900 is only good if Uncle Koru is still hiring as they are now. If it slows down in the future then you may be stuck flying a 1900 when you could be in line for a command on the Dash. Other airlines have taken FOs from both operators so if Air NZ is your goal then take what you can and get a Jetstar yes letter. Based on current Air NZ hiring practices you will be fast tracked for an interview.
In reality you should take what you are given and play the hand you are dealt as both options will place you better for the future.