I have done both. There are advantages to each. A mirror will connect me to a heritage of artists who gazed into their own eyes. They have a sort of haunted look of the close focus plane in the eyes that is very special. Selfies are great for capturing an expression and the type of distortion in them is fun to play with. I did my first selfie self portraits from photos on an old point and shoot film camera and that has a special characteristic too.