All of the tactical rigs are more or less modeled to resemble the real life tactical rigs, so realistically you would carry the same amount of ammunition/gear the same rigs can carry, meaning for example: 4 rifle mags and 2 pistol mags. Of course this is not entirely accurate for more modular vests, which you can customize your personal needs for combat, like the new rig the developers are creating.
Of course, for some raids it's cheaper to use the scav vest. But, if you want to raid for a longer period of time to loot as much as you can, so in advance you setup yourself to be better equipped, from head to toe.
This also depends on the amount of stash you have/don't mind to lose. Some players would protect their stash like gold, while others won't mind to lose a fully customized assault rifle which they probably spent a lot on it, as well as other combat gears and items.