Since this is happening in the city, instead of some huge open field, if Assad uses chemical weapons, he'll greatly increase the number of casualties of civilians who are loyal to him.
When an explosive detonates, those civilians who aren't supporting the rebels have some protection from the shrapnel because they're hiding indoors.
When a chemical is released, it can spread for blocks, seeping into the buildings through existing cracks or new holes made by shrapnel made by conventional ordnance and increasing the number of casualties in those people who aren't supporting the rebels.
If this were happening in an open field where only the two sides of the fight were present, I can't see any difference between explosive and chemical attacks. Here's a guess: Maybe the explosive attacks are more likely to be immediately lethal or have a higher chance of being healed, as opposed to chemicals which may be more likely to burn enough of a person's body that they can't enjoy life again ever, yet don't kill the victim outright.