Let’s look first of all at jumpers, or pullovers as they are called in some places. Jumpers are either on or off. If you’re cold, you put on a jumper. If you’re too hot, you take your jumper off. If you’re just a little cold, do you put it on or leave it off?
A cardigan on the other hand can be worn in different configurations. If you’re cold, put on a cardigan. If you’re still cold, do up some buttons. If you’re still cold, do up more buttons. Too hot? Undo some buttons or wear it open.
The versatile cardigan wins hands down over the jumper.
Or, as I like to say: jumpers are binary, cardigans are analogue.