No one, especially a Christian, can ever choose evil, however small. Everyone, especially a Christian, must choose good however small. In my opinion the evil in both Hillary and Donald so far outweighs the good in them it is impossible for a anyone, especially a Christian, to vote for either of them. Instead, write in the name of a person you judge to be more good than evil. It does not matter that that person stands no chance of being elected, but at least you, before God, have not chosen evil instead of good and that is pleasing to God who from the beginning of time gave us humans the power to determine for ourselves what is good and what is evil, but expected us to be guided by the natural law he impressed on our humanity. If we make our choices without reference to what Jesus Christ has taught us, we are no different than our first parents in choosing to call evil good and good evil.
Some would argue that when faced with an important situation we are allowed to choose the lesser of two evils. But the Church magisterium has always taught that one can only choose the lesser of two evils in cases of ABSOLUTE NECESSITY, such as whether to save the mother or the child in cases of an ectopic pregnancy. In the present case no person can say that IT IS ABSOLUTELY NECESSARY THAT THEY VOTE FOR EITHER DONALD TRUMP OR HILLARY CLINTON. Yes, Hillary Clinton favors abortion which kills unborn children, but Donald Trump will (in my opinion) cause America to go to war which will kill not only unborn children but men, women, boys and girls AND UNBORN CHILDREN. I do not believe that any human person can decide which of the two, Clinton or Trump, is the LESSER of the two evils. They are both so evil in their actions that no Christian (especially Catholics who have the benefit of the magisterial teaching of the Church, e.g. VERITATIS SPLENDOR) can play God and decide which is the lesser of the two evils.
What is important is how they must answer to God for their choice. My advice is for a perplexed Catholic to vote for all other offices on the ballot but to abstain from indicating a choice for President. If a Catholic is not happy with that solution, then that Catholic should write in the name of any good person for President.