On the JQd web site when you are selecting your fare you can hover your mouse on the little "i" icon with the flight times and if its an A320 flight it will say its Aircaft Type: A320, if its blank I would assume its an A321 (noe JQ A330's MEL-CNS). If you want a better chance of being less crowded I would pick an A321 if I could, 3.5hours in a JQ A320 with 29" seat pitch is really pushing the limit for my sanity as well, so I know where you are coming from.
Sometimes the QF or VA flights via SYD or BNE are only $20-$40 more than the cheapest JQ flight so you would be pretty silly to not consider QF or VA if that were the case on the dates that you were looking at. Especially as the cheapest JQ flights are either OMG o'clock in the morning or night flights. And on QF you have a chance of being fed/entertained and with VA and QF you earn some SC at least.
Oh - and I forgot that Tigerair now fly direct about 2-3 times a day from MEL-CNS (an A320 with 30" seat pitch) and are nearly always cheaper than Jetstar anyway, so you have absolutely no reason to book JQ at all, unless you can stick it to JQ and make JQ price match a Tigerair fare. There are various threads around on how to do that....
Watch out for the baggage and credit card fees as they may make a bit of a difference. I think TT and JQ will charge extra for exit rows from memory.....