Having a variable loan will give you the option of changing banks in the future if BW say they don’t want to give you loan for investment property. The benefit is it will keep you more flexible.

By the way, I would beware of pre-approval from Bankwest. They wont actually assess your finances (income, expenses, assets, liabilities & credit file) till you go for full approval. In my experience their pre-approval don’t count for much.