Some people prefer to solder their motors directly to the ESC, and avoid using bullet connectors. This help reduce additional weights, and possible electrical resistance. However by using gold bullet connectors, things get more flexible as you can swap motors, ESC without any more soldering.
I personally prefer using gold bullet connectors on my bigger builds (450+), because I like “plug and play” style and I can carry the little extra weight. If ESC or motor goes faulty, I can just swap it out easily, or if I want to change the spinning direction of the motor, I can just swap two of the motor cables without the need of soldering.
There are many different sizes of bullet connectors, depends on the current rating and size of the motor/ESC wires. When choosing the kind of gold bullet connector for your copter, make sure the connector meets your electrical rating. I follow these rules.
2mm connectors - up to 20 Amps 3.5mm connectors - up to 40Amps XT60 connectors - up to 60Amps 4mm connectors - up to 70 Amps 6mm connectors - up to 120 Amps 8mm connectors - up to 200 Amps
2mm and 3.5mm bullet connectors are the most common and cheapest, and they are suitable for most of the hobbyist level quadcopters and planes. Apart from the current rating, I always look at the size of the wires too. Always choose the ones that fit your wires nicely. If you have to remove some of the strand in the wire your using to small of a connector.