The first thing to consider is the size of the cables currently on the golf cart. Stock cables are normally 6 gauge. If you are only replacing the cables between the batteries, then you want to match the size of the rest of the cables on the golf car to prevent bottle-necking power due to a size reduction in cable.
If you are going to replace all of the power cables and you have or are considering any upgrade that will increase the amp flow from the batteries, you should upgrade your system to the 4 gauge cable. Upgrades that demand a higher amp flow are anything that will increase the speed, power or weight of the golf car or the size of the tires. So speed kits, torque kits, lift kits, big tires and wheels, ect. all require you to upgrade the stock wiring of the golf cart. This helps overheating and can prevent melting of battery terminals.