Sydney trip!

I'm super excited for my upcoming Sydney trip for 9 days with my friends.
We've made our itinerary from day 1 til the day we will be leaving the city.
This will be my second time ever in my life i'll be visiting Sydney.
VERY EXCITED!!!

We are planning to go to the GREAT Blue Mountains...
Here's the problem, I'm in need of an advise.
We are not sure whether to drive there or to just simply take the tour.
How much would it cost to rent a car? Would it costs us more than a tour?

From my research, joining the Blue Diamonds tour will cost AUD175.
There will be 4 of us!

I'm in need of a great help from everyone who had the experience (a day trip)
Both foreigners and Australians are welcome to have their input.

Thank you so much :)

P.S: We will be staying at Bellevue Hills area, which car rental company is the best?

X