2x8Gb vs. 2x4Gb upgrade for 2x4Gb iMac

Hi,

I would like to upgrade my iMac (Retina 5K, 27-inch, Late 2015), add more RAM. Since its a bit pricy for me to go full 32Gb upgrade atm, I am trying to decide on the configuration of DIMMs to fill in the empty slots. I kinda want to go for 24Gb to be safe, but I am not sure that it is a zero downside solution. Google search on the subject was rather confusing for me, and Apple support advice ('go for 2x4GB, to ensure [some technical shoo-shoo]') was the opposite of what the the service centre recommended by them suggested to me ('go for 2x8GB, its totally fine'), hence my question:

Is there any downside to adding 2x8Gb DIMMs to existing 2x4Gb compared to 4x4Gb configuration? What would that be precisely?

Thanks in advance for any help!