This is a question that has been asked many, many times. Research other posts on this subject and you will find a wealth of information.
Both Artisi and BigInch have given you good advice given the very limited information you provided. ANSI would be less expensive to purchase and install. API can be more reliable in high temperature, high speed, and hazardous services. But, consider the hazard carefully. Our sour water systems, in an oil refinery can be saturated with H2S. We are not allowed to work on these pumps without supplied fresh air. In a refinery sour water application with enough H2S to qualify as immediately dangerous to life and health, I would probably choose API. In a sour water application with much lower hazard and relatively low temperature, I would strongly consider ANSI. If you could provide more details about your application, we could provide more specific replies.
Johnny Pellin