Alois- Alois's emotions are quite unpredictable; one of his few consistent traits is his attachment to his butler, Claude, whose attention he is frequently trying to attract. He seems to have an overall cheerful disposition in spite of hardships and misgivings. Alois can be easily frightened (of such things as the dark, being alone, adding to the childish element of his personality) and is somewhat hyperactive. He shows signs of sadism and narcissism, as shown when he gouges out Hannah's eye in the first episode, merely because she looked at him while serving him. Alois seems to suffer from both bi-polar disorder and reactive attachment disorder. He has severe moodswings, going from a happy and pleasent child one moment and furious the next. His horrific childhood that contained no parental figures seems to spark his extreme need for affection. This is perhaps why he is so needy of Claude up until the very end.He often keeps low and underestimates the others as he thinks he is the greatest.His nature has been shown to act in an almost sexual, seductive manner to some of the other males in the series.
Sebastian- Sebastian is a dutiful and dedicated butler to Ciel. He follows each order faithfully, while always ensuring that the Phantomhive house runs smoothly. As such, he is noticeably annoyed whenever they end up behind schedule. The way he responds to Ciel's demands is quite notable, as the servants are pleased when they can successfully imitate the way he says, "Yes, my lord," to Ciel.
Despite this, he does not appear to truly care about Ciel or his well-being, as he frequently teases Ciel  and occasionally puts his life in danger for his own amusement. An additional example of this is when he responded to Ciel's kidnapping as "terrible," but only because the tea he prepared would go to waste. He calls Ciel a "brat" who treats his workers rudely. However, despite all the troubles he goes through on a daily basis, he has said that he doesn't think it's such a bad way of life. In Kuroshitsuji II, his personality has changed dramatically. Initially he saw Ciel as a meal and did not care if Ciel could or would get hurt. In the second season he now treats Ciel as a person and becomes extremely protective of him when he is endangered, and his "unusual" behavior has been commented on by Claude Faustus, who notices that the demon is 'abnormally attached' to Ciel, something highly unusual for their kind.
So there you have it. ^^ you could choose to play both if you can keep up with the characters. but u should choose the character i which you can rp better :D
All information gotten from: http://kuroshitsuji.wikia.com/wiki/Sebastian_Michaelis http://kuroshitsuji.wikia.com/wiki/Alois_Trancy