Certification means the subject passed a minimal requirements standard managed by a recognized oversight body. "attesting the truth of some statement or event."

Designation means the subject passed a financial capability requirement (paid for it, and keeps on paying for it) In this application regarding REALTOR designations, it is implied that oversight is present through a recognized body (Local REALTOR board) but does not 'attest' to any level of proficiency.

I am no longer sure. In my area, many realtors add esq, mba(stanford), phd or their mortgage and appraiser license # and a CPA #.   I know someone who addes Certified inspector, licensed structure engineer plus his broker license on their calling card. Many wear several hats.

Designations typically result in annual fees to keep them

Nothing. It is not the certification nor the designation that ultimately makes the clients come back. It is the persona of the licensed real estate agent that he liked. However, one's ego would like to have both at the end of his name; but, designation might be more impactful. 

Not a whole lot.  I have NEVER been asked for my designations or certifications.

A designation carries more weight.  

Certification is usually a simpler set of standards to achieve and once you reach them you can use the certification in you advertising.

A designation has more education or levels of achievement with it. In NAR you have to continue to belong to the council associated with the designation to be able to use it in you advertising. If you drop out of the society or council you cannot use the designation any longer.

Annette Lawrence has this down..the specifics..they are equally important.

Not sure other than maybe an onetime vs annual fee.

probably nothing! It's a piece of paper...