Both. It depends on complexity of the kit, how many colours, moving parts, stuff like that. The best way is always the way that you think would work best. There is no right or wrong answer, it always depends on the situation.
But usually you would make parts like legs, arms and torso separate, paint then and then put the model together instead of putting the model together and then painting it, even if it is all the same colour. This way the paint is more consistent and gets in the spaces which would be covered by the panels of the skirt or shoulders. Otherwise if you paint it and then change the pose it can reveal unpainted areas which is very amateur in contests and also limits what you can do on your shelf when posing.
