Extra virgin is from the first pressing of the olives and contains the most beneficial qualities. It is usually more expensive and stronger tasting, but most of us use it because its the best. Extra light oil is from the third pressing of the olives and may contain other oils to stabilize it (according to some sources). I have used it in the past and it worked, but,it also caused more nausea than the extra virgin.
I hope I'm remembering all this correctly, its been a while since I've studied the oils. Still, when flushing, go for the best you can afford. Your liver is worth it.