Prospero generally greets Caliban and Ariel with the words 'slave' and 'servant'. Prospero calls Ariel the servant and rarely uses Ariel's name. Prospero also calls Ariel 'spirit'. It is quite apparent that Prospero feels that he owns Ariel, always calling Ariel 'My brave spirit' and 'Why that's my spirit' using 'my' in these sentences when he speaks to Ariel, showing possession. This is reflected in what Ariel calls Prospero - 'master'. Prospero is obviously fonder of Ariel than he is of Caliban.
Prospero often slips in a kind or complementary word when speaking to Ariel, such as 'Spirit, fine spirit' however, Prospero only ever speaks to Caliban in a harsh and cruel way, the first greeting in the play to Caliban is 'Thou poisonous slave, got by the devil himself upon thy wicked dam, come forth'. So it is not even like Prospero just commands Caliban, he has to go into the cruel detail of how much he dislikes him. 'You poisonous slave' not only is he a slave, but he is so disgusting that he is poisonous too. 'Got by the Devil himself upon the wicked dam' in this sentence, not only is Prospero saying that Caliban's father is the Devil himself, but he is also saying that his mother (the dam) is wicked and bad and the cruellest she could be. The cruelness that Prospero shows to Caliban may be because Prospero is revolted by Caliban, but it may also be because Prospero knows that he cannot treat Ariel in the same way as he treats Caliban as Ariel does things for him and he therefore wants to keep Ariel sweet. Prospero also congratulates Ariel when he has done something for Prospero.
Prospero gives out compliments such as 'Spirit, fine spirit' and 'Delicate Ariel' whilst Caliban's efforts are often overlooked by Prospero. Prospero treats neither Ariel nor Caliban well, but it is apparent from the text that that Ariel is th best treated of the two. This is because he does have a certain amount of power over Prospero due to his powers that Prospero desires.