Stage lengths in New Zealand are too short to see much benefits from the speed of a jet I think.
10 minutes time saved at the most, and the majority of pax would rather have $10 lower ticket than 10 minutes lower time.
General worldwide trend for sure, and New Zealand is not the place for an airline to buck the trend in this sort of way.
With regards to fuel prices, I think this is quite telling -
this is an example from Airliners.Net:
"a 1 hr trip on an RJ will burn 2,200 lbs of fuel. That same trip will take 1.1 hours on a prop and burn 1,650 lbs of fuel, or 550 lbs less, despite a longer trip time. That is a 25% reduction in fuel cost per trip, which is significant."
I'm not sure how accurate that is, but still.