There's a discussion in the other topic, but it really comes down to usage. If you use a lot of electricity, but not using much between 4-9pm, then Prime is better.
Domestic:
Daily allocation for Irvine is 11.4-12.6kWh, so your monthly baseline is about 340-380kWh. This would be billed at 26c, then 101-400% will be billed at 33c.
Prime:
off peak rate is 19c/kWh, and 48c/kWh 4-9pm. And there's also a daily surcharge of $0.40, so add $12 on top of the usage.
There is also the TD-4-9PM:
off peak rate of 30c/kWh, and 47c/kWh 4-9pm. But you get a baseline credit of 8c/kWh, and based on the 340-380kWh baseline, that's about $30 rebate.
You can look at your last month's usage and figure out what's best for you.