"bigger" is only one aspect to look at. Which shape and dimensions fit the locker cavity best. Just because the manufacturer states the bags holds X, does not mean you will get that amount of water in it when its in the locker. As to fittings, if you are plumbing the sacs in, you might as well say neither come with fittings. The Fly come with what you need for mating with their W709 pump kit. The other place, all fittings are a-la-carte. As to the larger fittings. Its a minor difference with aerator pumps and no difference with impeller pumps.

Given flu high's track record, I dont think material thickness is a factor.