Should I buy a car in USA or in Mexico?

You would be living practically in the U.S. so import would not be a problem, almost everywhere else it would. You will be going back and forth so it wouldn't really matter, your choice. I have had no problem with the gas in Mexico. Be careful with insurance. Try to investigate a local agent on the Mexican side. Someone you can easily get in contact with. Mexican insurance is not a requirement I am told, but given  traffic it would be advisable. We had insurance in the U.S. and in Mexico, given that the U.S. as far as I know does not work in Mexico and likely vise versa. Your visa status might be a factor in your decision.

Thank you for the reply.  I would certainly prefer buying the car in San Diego.  The thought of having US insurance for a couple of days a months is troublesome.  We will probably be on a Tourist Visa and using the California drivers license.  Any other ideas out there?  Insurance agent recommendations in Rosarita or Tijuana?