Both Plato and Aristotle would likely be opposed to abortion, based on The Republic and Nicomachean Ethics, respectively. There are ideas in Aristotle's ethical arguments which might allow more leeway for the act of abortion than is found in Plato, but in general the great weight of both philosophers' arguments would seem to be against abortion as not morally permissible.
Neither man discussed abortion directly, which makes this study speculative. Considering Aristotle's golden mean, and his greater emphasis on freedom for the individual as opposed to the power of the state, one might find room for some behavior which would not be permissible in the more strict moral world of Plato as described by Socrates. In addition, a feature of Aristotle's moral philosophy is the tenet that every individual believes his or her action is for the best, that it will somehow advance goodness: "Every action and pursuit is thought to aim at some good; and for this reason the good has rightly been declared to be that at which all things aim" (Aristotle 499).
It appears that such an idea would lead to the conclusion that everything is permissible because everything leads to the good, but that is obviously not what Aristotle means. In fact, Aristotle differentiates between acts which the actor believes to be good and those that are in fact good. The woman who seeks an abortion might believe she is seeking the best for herself and the unborn fetus. However, even if that were true, Aristotle, like Plato, is designing a moral philosophy which considers not what is good for the individual alone but what is good for the community:
For even if the end is the same for a single man and for a state, that of the state seems at all events something greater and more complete whether to attain or to preserve; though it is worth while to attain the end merely for one man, it is finer and more godlike to attain it for a nation or for city-states (Aristotl...