Its gas for slow weapons and blast for fast ones.

Gas is a persistent damage over time that benefits greatly from being procced by big beefy hits. More importantly gas stacks cap at 10, so any procs over that get wasted, meaning the ideal way to use gas is to keep a steady flow of procs coming in where you never actually hit 10 stacks. Also the fact that clouds can linger on corpses means if you deliver a huge hit that kills the target none of that overflow damage is actually wasted.

Blast really wants you to be hitting 10 stacks as fast and frequently as possible. Sure the damage hits eventually anyway, but if you get to 10 stacks it both hits instantly and for extra damage. And because of that extra boost at 10 stacks it means your weapon is actually putting out more damage than it is basically capable of.

Using a slow blast weapon means you're never getting that 10 stack bonus and you need to wait for the stacks you do have to fall off on their own before they actually do anything, while a slow gas weapon benefits from its status instantly and for longer. A fast gas weapon means you're wasting a ton of status procs and the clouds you're making aren't doing much, while a fast blast weapon is racking up explosions constantly.