Sludge blast/VIM

Ok its been a few years for all the grey beards to tinker....what is the argument for using sludge blast over Volatile impact mixture on sludge pump? I only take either of these on dreadnaught mission atm. Is there other mission types where they can function servicably? The low ammo is the kicker, and I doubt sludge blast could deal with swarms well. They both are so similar in the damage/ play style. VIM has the option for normal shots or big charge shots vs sludge blast only being charge shots ext. Anyone an expert using both?