One thing I can say for sure is you don't want me to do it. I started on mine over four years ago and it's still not finished. The whole thing was supposed to take six months.
Bwahahahahahaha......amateur! Building permit approved in 2007, still going. Don't even have the master bathroom plastered yet.
If you're going cookie cutter, basic box, then the big boys will be fine. Once you start getting bespoke, they'll either fuck it, refuse to do it, or charge you through the arse for it. Even basic things like square set corners have them raping you.
Going private is better, you'll have more input during the build, variations won't be such a big deal/sticking point and the attention to detail will be miles above Hillies and their ilk. However finding a good builder is tattslotto, thanks to the property bubble, and there's a lot of fly-by-nighters out there. When we started half the builders we contacted quoted a by square price over the phone, without even looking at a plan.
And for the love of all that is unholy,
DO NOT FUCK ING PROJECT MANAGE IT YOURSELF TO TRY AND SAVE A FEW BUCKS! It will cost you, both time and money, not to mention your sanity and relationship.