I'm not sure if United would have a clear preference.
United lost 6 in a row before beating Perth recently, and lost 10 of 11 against NZ before beating them twice in short succession last week.
Those 6 losses to Perth were mostly by very small margins, whereas I can't recall but I suspect some of the NZ ones were more comprehensive losses with some very close losses there too.
Either way, the point is United hasn't been successful against either team over the past season or two.
In my mind, Perth is still the more dangerous outfit because their best is better. They have a higher ceiling than the Breakers IMO. Cotton is better than anyone on the Breakers roster and if the team gets back in form, they are the more worrying opponent.
But- it doesn't matter. United has to believe they are good enough to beat anyone.