I would generally avoid Emeritus, since many are not taking students or are winding down their careers. You want an advisor who is still very active in their field. However, some emeritus are still very active and even have *more* time for students because they don't teach. So, if you are very interested in a particular emeritus professor, I would email them to find out if they are taking students. Also, check to see that they are still participating in conferences and publishing papers.