It depends. On your timing, needs and whether you are carrier limited because of where you live. Wisdom says it is better to purchase outright there may still be exceptions.
Past experience with Telstra for business, the phone was always a full cost add on. There was always more choice if you purchased separately and better value. I now go month to month for the cheapest plan I can get, as it is really just a backup for travel out in the yonder. The phone is ancient, but works.
Optus is the only service we can get reliably in our house!
With one exception we have purchased three out of four previous phones out right. Typically from a phone specialist. Also we have paid month to month as this keeps up with the changing offers.
Unusually my current mobile was purchased as part of a plan. There was a big push to get the iPhone 8 out the door, with slow sales and the X on the way. Based on a current rrp of just short of $1,000 I have a plan phone included ($59 rounded) that provides un limited calls, SMS and 20GB of data for less than $17pm if you take the phone rrp out. The phone is effectively full rrp paid over 24 months.
This deal is long past, however it does illustrate it can be worthwhile looking at all the options. Alternately looking at similar BYO plans from Optus at that time the phone was less than half price. My iPhone 4s was still going after 6 years, but had dropped off the IOS and security update support cycle.
Two of our grown up offspring also buy outright as they believe it is cheaper and gives more flexibility with carriers and plans. Our other more cash strapped one always goes with a plan.
A phone with a plan is just another line of credit for many of us, but you can find the occasional better deal?