Since you are asking about a certificate in nutrition I will assume you know that Registered Dieticians(RD) is a degree program with a national certification from the ADA and not a nutrition certificate.
Since you have been training for 8 years I will assume you have a base knowledge about nutrition and are simply looking for a way to tell your customers you have a certificate/knowledge? In this instance you need to find out what laws your state has in place to become a certified nutritionist. Most states do not have laws and the title "nutritionist" is unregulated. In some cases, when training at a national-gym-chain the gym itself will have rules about what you can and can not advice your clients to do so look into that depending on where you work. One last thing to point out is that "some" trainers carry insurance for liability reasons and the insurance may stipulate what you can/can't tell your clients so look into that too.
If you find yourself in a situation where your state, club, and insurance carrier do not care that you tell your clients what to ingest then get whatever certificate you want. People on here may give you opinions about what certificate is the best but, A) these people won't be your clients, and B) your clients won't know a damn thing about what a certificate means when you list it on your bio or display it in your office.