We like the quick release. We are planning to get one for each of our bikes. Our son (9 years old) also bike with us with no bag. We want to pack stuff for day trips - extra water, snacks, light coats, wallet, extra tube/levers, etc.
Rans Enduro Sport, Hase Kettweisel Tandem, Merin Bear Valley beater bike
Trunk bags range in size from about 400 cu in to around 750 cu in so this one is on the smallish size. It's big enough for a rolled up rain jacket, a spare inner tube, wallet and not much else. Once you make the committment to carry one on your bike, you'll keep finding new tools and such that you didn't previously realize were necessary to carry with you.
The big issue with seatpost mounted racks is where they fit on the seatpost. You've already got suspension seatposts so that takes up a lot of the exposed mounting area. Make sure that you've got a couple inches of available space up high enough that your rack won't be hitting the brake arms or rear tire.