The most well known difference between Aristotle and Plato (which makes it in all likelihood the most well known philosophical difference ever) is the disagreement about the reality of universal forms. The difference can be put in several ways, but the way it most often shows up in Platonist literature is as a disagreement about how we are to understand a series like [man, animal, living, being]. For the Platonists, this series goes from what is less real to what is more real, since each step ascends to a subsistent form that is responsible for more things; for the Aristotelians, the series goes from what is more real to what is less so, since it is an ascent of predicates that are more and more potential. Raphael captured the difference in the central scene in his School of Athens, which shows Plato pointing up (towards more and more universal form-predicates) and Aristotle pointing down (towards the concrete individuals).
St.Thomas, while remaining basically Aristotelian, sought to form a synthesis of the two views. His doctrine of ideas being forms in the divine mind is well known, as is his ideas of participation (St. Thomas insists throughout his career that Aristotle accepts and advances a doctrine of participation). But the only time that St. Thomas makes a clear synthesis of the two views as complementary accounts of a series of predicates is in a short response to an objection in the Prima Secundae:
The difference compares to the genus as form to matter, inasmuch as it makes the genus to exist in act. But the genus is considered more formal than the species, inasmuch as it is more absolute and less contracted. So the parts of the definition are reduced to the genus of formal cause, as is said in the second book of [Aristotle’s] Physics. In this way, genus is the formal cause of the species, and in the measure that it is more common, it is more formal.
differentia comparatur ad genus ut forma ad materiam, inquantum facit esse genus in actu. Sed etiam genus consideratur ut formalius specie, secundum quod est absolutius, et minus contractum. Unde et partes definitionis reducuntur ad genus causae formalis, ut dicitur in libro Physic. Et secundum hoc, genus est causa formalis speciei, et tanto erit formalius, quanto communius.
Not only does St. Thomas say that there is a sense in which the more universal predicate is more formal or actual, he cites Aristotle himself in support of the opinion!