I think the question of who's more pure in singing depends on the person who sings and the one who listens to it. If a countertenor sings with a mixed choir, his voice might stick out from the group although this would depend on the conductor. If he/she is a good conductor, then he/she could make that person blend with the group. On the part of an alto, you couldn't just mix them to an all male choir because this will not sound good. So this just means that both countertenor and alto have differences. There is a thin line of distinction and limitation to where their voices belong. So the question of who sings more "pure" or who sings best depends because there are countertenors and altos who are good. We must remember that a male voice is different from a female's voice.