less vs not so much

less

• unless 

• Minus; not including 

• A smaller amount or quantity. 

• Fewer; a smaller number of. 

• A smaller amount of; not as much. 

• To a smaller extent or degree. 

• Used for constructing syntactic diminutive comparatives of adjectives and adverbs. 

not so much

• Indicates that the speaker identifies with a prior statement to a much lesser degree (or not at all) than the other person. 

• Indicates that one factor is much more relevant or important than another.