Which is better? l-carnitine or acetyl-l-carnitine?

Synonyms: Acetyl-l-carnitine and l-carnitine are synonyms for the same substance, but you want to make sure to have the "l" form and not the "d". Make sure the producer specifies this, as well as any % content of "d" impurities. In humans it's synthesized in liver, kidney, and brain from lysine and methionine and actively transported to other areas of the body -- 98% in skeletal and heart muscle.