Who better than the Ven. Bhikkhu Bodhi to discuss the competing Buddhist ideals of the arahant and the bodhisattva? Bhikkhu Bodhi has been trained in both the Mahayana and Theravada traditions and is one of the most respected and, um, thorough Buddhist scholars around. His paper "Arahants, Bodhisattvas, and Buddhas" appears on Access to Insight. The introduction is reproduced below, but it's worth reading in full on ATI. Among other interesting points, he discusses what distinguishes the Buddha from other arahants, and describes the emergence of the Mahayana from a proto bodhisattva-yana.
The arahant ideal and the bodhisattva ideal are often considered the respective guiding ideals of Theravāda Buddhism and Mahāyāna Buddhism. This assumption is not entirely correct, for the Theravāda tradition has absorbed the bodhisattva ideal into its framework and thus recognizes the validity of both arahantship and Buddhahood as objects of aspiration. It would therefore be more accurate to say that the arahant ideal and the bodhisattva ideal are the respective guiding ideals of Early Buddhism and Mahāyāna Buddhism. By "Early Buddhism" I do not mean the same thing as Theravāda Buddhism that exists in the countries of southern Asia. I mean the type of Buddhism embodied in the archaic Nikāyas of Theravāda Buddhism and in the corresponding texts of other schools of Indian Buddhism that did not survive the general destruction of Buddhism in India.
It is important to recognize that these ideals, in the forms that they have come down to us, originate from different bodies of literature stemming from different periods in the historical development of Buddhism. If we don't take this fact into account and simply compare these two ideals as described in Buddhist canonical texts, we might assume that the two were originally expounded by the historical Buddha himself, and we might then suppose that the Buddha — living and teaching in the Ganges plain in the 5th century B.C. — offered his followers a choice between them, as if to say: "This is the arahant ideal, which has such and such features; and that is the bodhisattva ideal, which has such and such features. Choose whichever one you like." The Mahāyāna sūtras, such as the Mahāprajñā-pāramitā Sūtra and the Saddharmapuṇḍarīka Sūtra (the Lotus Sūtra), give the impression that the Buddha did teach both ideals. Such sūtras, however, certainly are not archaic. To the contrary, they are relatively late attempts to schematize the different types of Buddhist practice that had evolved over a period of roughly four hundred years after the Buddha's parinirvāṇa.