I feel that you want to know which is more malefic dasha period. The Dasha of 12th house or the dasha of the planet placed in the 12th house. The dasha of the 12th house is always more malefic than of the planets posited in the 12th house. 12th lord dasha is more malefic if there is Mooltrikona sign in the 12th house. The worst dasha is the 8th house dasha and second to it is 12th lord dasha. The planet if posited in the 12th house will become weak but if it is benefic it will still give positive effects but can be less. The 12th dasha will be malefic as 12th lord is a functional malefic in the horoscope.