US MJ is better because it is carefully cultivated to provide various properties that can be felt by the user. While some are using it to be high, others find it can reduce pain levels safely, unlike deadly opioids, or reduce depression. There have been news reports saying that US MJ is better quality than Mexican and sales are going down for the Mexican import. I have friends living in legal states that report benefits like less fatigue, reduced pain from arthritis in hands and spine, reduced depression after death of 2 elderly pets. Some varieties are more effective for getting high, others have more medicinal actions. Most people I know who are using are responsible members of society who choose and use their MJ in a responsible manner. Since one can legally go to a medical dispensary, or get free material from friends, you can be much more sure of the purity of the product and the information on strength and dosage.
This link answers your question in much more detail. Included is the fact that wealthy Mexicans are getting imported US MJ.
https://www.cbsnews.com/news/what-do-marijuana-smokers-like-better-mexican-or-american/
Here is a more complex report on what is happening in US versus Mexico regarding MJ. Also many comments.