The 717 and the 146 burn around the same amount per hour.
Don't know what the tankage is in the 717, but the limiting factor in the 146 in many cases is the 9000 odd kg's that the tanks will hold.....if the 717 holds more it may be a different kettle of fish.
If the decision is not made until after Christmas, Jetstar would have to be loking good, though I suspect that NJS would have to be in a good position for a range of logistical reasons.