Don't forget there are also weight limitations on carry on bags, as well as size restrictions.

If you're flying domestically in the US, the requirements are fairly generous (around 40-50 lbs). However, international flights have much stricter weight limits, sometimes <20 lbs, and even some as low as 15 lbs. This will make a big difference in the type of bag you choose, as some bags weigh almost 8 lbs empty. As far as I know, most airlines aren't enforcing these limits very strictly, but at one time they also didn't use to be so strict on the size limit either. It's really up to the discretion of the screener.

One way to avoid this is (if you're not checking bags) to wear all your heavy and bulky clothes (boots and jackets) through the screening process, then pack it back in the bag when you're through. So far, there's no limitation to how much you can carry on yourself.