The option to buy premium ammunition for gold is available for the majority of tanks in World of Tanks. These shells generally add increased penetration values and some will add different damage values in exchange for other benefits. These shells are usually HEAT, HESH, APCR, and APDS rounds and are available for purchase for gold or credits. The gold option makes using these shells very expensive and wasteful since you are literally throwing money at people for small gains.
The credit option however is the better option(it follows the 400:1 credit:gold ratio) since you are not spending actual money. These shells are typically used in large numbers for competitive player but in public matches should be used a bit less. Typically carrying 15%-25% premium rounds in public matches to deal with heavily armored tanks will fill most player’s needs. Since they are expensive you only want to use them against tanks you cannot penetrate reliably, but when you do use them they will increase your chance of doing damage and winning which offsets the increased cost slightly.