If it were me I'd go for an earlier time as there's no way on earth Jetstar would compensate you for you LA flight should you miss it. Would Virgin Blue? I don't think they would either.
I used them twice when in Oz, however, and found them to be fine. (Think Easyjet, if you're a European!!)
My biggest concern is not catching my connecting flight which will be leaving at 4:25pm from Sydney to LA. The flight i'm looking at on Jetstar lands in Sydney at 3:10pm. I HAVE to make sure i catch that connecting flight, as i am due at work the following day. Not sure if its work taking a chance or if i should just book with VirginBlue. Their flight is $195 and this one with Jetstar is $169. Maybe i'll just suck it up and pay the extra few dollars.
Forget about the Jetstar flight then you will definitely miss your international flight check in as most international checkin counters close 60 minutes prior to departure and in Sydney the domestic terminal is in a different building to the international. Jetstar is also a point to point airline meaning they will not transfer your baggage to your flight so you will have to collect your baggage and recheck it in. If Virgin Blue leaves a few hours earlier get that otherwise if it is very important to get to Sydney on time I personally would fly to Sydney the day before and spend the night there as both go in to a different terminal to where you'd get your international and both would require you to collect your bag get the bus or train to the international terminal and re check in which takes probably 25 minutes or maybe more depending on how long your bag takes to come out and how big the queue is. Then you have the huge queue proceeding through immigrations in Sydney that takes ages to progress through to get stamped out of the country.