If you need more then 16GB of RAM at any given time then use 32GB.
If you dont use more then 16GB at any given time and If this is a Ryzen PC then 3000mhz would be waaay more beneficial. With Intel although there will be a performance hit it wont be nearly as bad so It really would be up to you if you want to use 32 or 16.
Remember having more RAM does nothing for performance if you arent using all the RAM.