Between NZ and AU regulators. AU is a lot stricter, recent modifiions in ASIC regulations dictate that brokers require insurance policy on clients funds. (demand CS to send a copy of their insurance coverage).

With respect to slippage and spread and execution pricing, nothing is perfect. Its hard to tell because it mostly depends on market conditions/aggregator. One that slip now, might not slip tomorrow vice versa. Same thing goes for spread.
U might want to start cent/demo accounts with of the brokers have shortlisted and use this to find the one that is most consistent to other brokerages. https://tradeproofer.com/