The "Illiad" is built around the charismatic but hot-headed young warrior Achilles and how he reacts to people and events around him. Although Aenes is a formidable Trojan warrior his character is less prominent than the Trojan brothers Hector and Paris as the former is killed by Achilles and the latter kills Achilles. Aeneas emerges as the protagonist in the "Aenead" as he leads the Trojan survivors of the war on a dangerous sea voyage that eventually takes them to Italy.
Homer uses Achilles as an example of the archetypal warrior who is brave, strong and feared by his enemies. However, Achilles is also an example of a haughty and arrogant man who sometimes lets his emotions rule him to his and his comrades' detriment. Virgil uses Aeneas as a more selfless hero set upon a journey to help preserve something of his people and culture. While Aeneas is not always virtuous, he is much less prone to the selfish and immature behavior that Achilles exhibits.
Until his death near the end of the "Illiad," Achilles remained uninjured, but for long periods he refused to fight the Trojans because of a dispute with the commander of the Greek forces. He was later compelled to action after the death of his best friend and exacted revenge on the Trojan hero Hector. Aeneas acted honorably during the war and was only out of action when he was wounded in battle. He carried out his duties like a professional soldier unlike Achilles who put himself before the other Greeks.
Achilles was aware of a prophecy that foretold his death if he sailed from Greece to Troy to do battle. The prophecy also promised he would win honor and glory on the battlefield and would be remembered down the centuries, so he chose to go and fight instead of stay home and lead a comfortable life. Aeneas was not to be remembered primarily as a great fighter, but as an honorable savior of the survivors of Troy who would lead them to safety and help to build a new society in a new land that would one day be known as Rome.