I think that Sasuke will finish off Orochimaru but it won't be as easy as last time. Don't forget that Orochimaru was weak and his body was just about finished when Sasuke defeated him. Even Sasuke said that if Oro had been at his peak he wouldn't have won. I think it will be an extended battle this time. Besides he said he needed Oro to find the one who knows everything. Oro is expendable, and I don't think he is going to be Sasuke's teacher.