Indra and Purusha are both mentioned in the Vedas. Indra is depicted as a god of war and of storms and lightning. Purusha on the other hand is shown as a giant being whose sacrifice gives rise to all things and all beings in the world. The gods, the sun and moon, the four classes in society all come from the sacrifice of Purusha.
In the Vedic ethos, ritual is the supreme form of religious performance. Priests chanted hymns to the gods to receive their sacrifices. By the time the Upanishads were being formulated, forms of meditation and yoga were replacing sacrifice and ritual as the dominant religious activity. The goal became less one of maintaining the cyclic order of the world than one of finding the soul within, and finding that your true Self (atman) was rooted in the divine essence of the world, Brahman.
With this in mind we can make some contrasts between Indra and Purusha. Indra represents the older order, in which there are a multitude of gods. The priests perform the sacrifices, the gods perform their duties. Indra is king of the gods, so is seen as governing this order. On the other hand, since everything comes from the sacrifice of Purusha, we have an idea that more closely resembles the symbolism of the Upanishads: everything comes from the One. Everything is interrelated. And it is only through sacrifice, and symbolic self-sacrifice at that, that one experiences this oneness.