This is easy. Everybody has the right to end his or her own life. The doctor should never terminate that life even if requested, however. Abortion is not concerned with a person's own life, but rather another life.
I don't think it's so easy. There are number of ways to argue against this. First of all would be compassionate use of medicine--don't we, as physicians, have a responsibility to ease the pain of others? And what if their life itself is painful--do they have a right to terminate their own life with the comfort that modern medicine can provide? Do we as physicians (future physicians, really) have an obligation to care for this person, even if it goes against our moral framework?
On the other hand, we do take an oath to "do no harm." But where is more harm done--by abandoning the patient and denying care or by carrying out their wishes?
(FYI I'm not taking a particular stance on this issue--just presenting a feasible alternative argument)
Abortion is most definitely concerned with two people's lives--the main reason to terminate a pregnancy is it is undesired by one life (the mother). Personally I do not support abortions and will not perform them (but will notify someone seeking one of who can aid them--that is the responsibility of any physician), but to claim that the argument is as simple as you state it underscores the true depth and complexity of the issue. People disagree about abortion and euthanasia/physician-assisted-suicide precisely because they are so complex and incredibly hard to justify with certainty either way (to the extent that the majority of the public would support one over the other).
Also, I agree with an earlier poster that abortion is probably not ever the ethical thing to do. Nobody is ever pro-abortion--they are pro-choice. I don't think many would say killing/aborting a fetus is the ethical thing to do, but rather the ethical thing is to let the mother/parents decide. If the parents do see anything wrong with an abortion (ie, that it's not a life and that it isn't killing) then they are not saying an abortion is the ethical thing (or the right thing) to do, but rather that it is permissible and that they have the right to choose. That's why it's framed as a pro-choice vs. pro-life debate.
FYI, in my mind "ethical" means (in a very simplified manner) "what is right" and "what should be encouraged," and my argument is based on this definition. People who disagree with this definition, however, may agrue another story--and that's part of why ethics is so difficult--people define the same thing differently (such as when life begins).
I think the more difficult argument is not whether abortion should be allowed or not--everyone is pretty strongly on one side or the other. A more difficult argument would be are we morally obligated to abort a fetus who will with certainty be born with such birth defects such that their lives will be entirely without joy, and extremeley painful and miserable (perhaps they're born blind, deaf, extreme arthritis and chronic pain with no possiblility of control through medication, etc.). It may not be a very realistic/probable question, but I think it is a more difficult question, at least for pro-lifers, because it tests your sense of compassion (which can argue the case either way) and possible willingness to break from your normal conception of right/wrong. Our job is to preserve life, yet also to prevent or minimize suffering. What is our responsibility here? While some of us would be willing to or want the chance to live, despite any hardships, others of us would not want to live through such misery. Yet, despite what others feel, it doesn't tell us what that unborn child or fetus feels.
Maybe a good question for a pro-choice advocate would be to ask what is appropriate when the husband and father wants the baby to be born, but the mother wants to abort it (maybe because the child will be bown with Down Syndrome, to connect this with the previous question). Does the mother have the last say since it is her body, or does the father have the right to see his child born?
I think questions like these--ones that are a bit more tuned to what people already believe, get a better sense of the throught process of an individual. Really, the more unfamiliar the question, the better (for the adcom!!!)