I think the rule of thumb has been that if you are casting at 30' or less and require a delicate presentation, then a DT line would work well. More than 30' and where delicay is not an issue, then WF. Also if you are casting larger flys and or streamers, then you might also want to consider the WF line.
Not to complicate matters, but there are those that feel a DT line will mend better than a WF line.
" If a man is truly blessed, he returns home from fishing to the best catch of his life." Christopher Armour