Answer: Totally different experiences. Taronga is a large, traditional outdoors zoo. The Australian collection is very good and the zoo itself is quite scenic with great views across the harbor to the Opera House and Sydney. Getting there is easy via the ferry which is a fun trip in itself. I think I spent 4 hours at Taonga when I visited in November.
Wild Life is smaller and indoor. Many of the reviews indicate people thought it was way overpriced. It isn't cheap, but I wanted to see it anyway. I worked at having a very good experience. I was there when the doors opened, went on a non-holiday weekday and knew the times for all the zoo keeper talks. With the advance planning, I did enjoy the facility. The keeper talks were quite good and I had a chance to pet a wombat in their enclosure. The animals are in smaller enclosures than Taronga. I could get quite close, but I think Wildlife could easily become over-crowded and viewing would not be easy or enjoyable. I was at Wildlife Zoo for a couple hours which included all the morning keeper talks. I think most people spend less time. The route through the building leads you forward in one direction, but I did backtrack and returned to see the Tasi Devils who were hiding during my first pass by.
Koalas and 'roos at both facilities. It was the platypus that I found amazing!!! Enjoy, we see so few Australian animals in the US zoos, it was a treat!