I think that you may be misunderstanding/misinterpreting what UA is offering at $250

I think that that is only for Economy Plus seating, not for premium economy. Premium plus is just economy seats with extra legroom. Other airlines also offer the option to purchase extra legroom seating

Premium Economy is a different class of service altogether with wider seats and more legroom and larger recline. something more akin to domestic US flight 1st class seating than economy

If you travel on Qantas, you will be able to purchase extra legroom seating and will likely cost something similar to what United is charging for Economy Plus

I would start by price/schedule/conveniance - so would , based on what you say, be looking at Qantas or United

I would suggest pricing up both carriers based on cost of extra legroom seating on Qantas vs Economy plus on United