This question is difficult to answer in a few words here. A good overview of the Roman Catholic theology is available in Predestination by Garrigou-Lagrange. He concludes with a view similar to Aquinas: an effect may have multiple causes, both God's will and man's free will may cause man to make a specific choice.
Also, I have found C.S. Lewis explains this question rather well in Perelandra. His approach is by no means systematic, but for a lay philosopher or theologian (like myself) it is a useful, concise summary.
The whole struggle was over, and yet there seemed to have been no moment of victory. You might say, if you liked, that the power of choice had been simply set aside and an inflexible destiny substituted for it. On the other hand, you might say he had delivered from the rhetoric of his passions and had emerged in unassailable freedom. Ransom could not for the life of him, see any difference between these two statements. Predestination and freedom were apparently identical. He could no longer see any meaning in the many arguments he had heart on the subject.
