Achilles, the son of mortal Peleus and the goddess Thetis, is the focal point of Homer's Iliad. The name of Aeneas, the son of mortal Anchises and the goddess Venus, can be found in the title of Virgil's epic poem, the Aeneid. Although Virgil was influenced heavily by Homer, Virgil gives his hero different motivations.
Achilles is motivated by individual glory. Prophecy has told him that he can die young and glorious at Troy, or live to a ripe old age at home in Greece.
Aeneas is motivated by a national glory that awaits for the Roman race that he will help to found. After surviving the Trojan War, Aeneas sets out in search of a new homeland, a homeland that various prophecies have told him will be in Italy. Establishing that new home is the focus of most of what Aeneas does.
Achilles, in contrast, does not have to establish a new homeland or reclaim a homeland that has been lost. Achilles fights for glory and also fights, in the Iliad, to avenge the death of his comrade Patroclus at the hands of the Trojan Hector.
Aeneas has his own instance of vengeance. His ally Pallas was killed by Turnus and Aeneas, in the closing lines of the poem, avenges Pallas' death by killing Turnus.
As we can see, Achilles and Aeneas do share some similarities, but Achilles' quest for glory is an individual one, whereas Aeneas is more of a "team player". His motivation is the establishment of a nation whose glory will reach its height hundreds of years after his own death.