Im gonna go with Onii-Chan cause it just excites me more! :D
"Master"... Its like she doesnt have a choice in the matter (I.E shes not gonna be enjoying herself) it also makes me sound like a rich prick!
Another turn off for me is the fact that I know that id hate to be treated that way. To be a servant (Slave) and dedicate my whole being to that person.
Makes me feel a bit angry TBH and I couldnt put anyone through that, Its just the way my mind works.