Dorothy would go back in time, without question. She has spent her days since the indecent at the diner wishing things could have gone differently, and blaming herself for all of the terrible outcomes that emanated from that day.
She has also spent her life, day in and out looking to the future in her work,and would focus on it, more than enjoying the present. She would go back to see and feel those days that she ignored in the past, and experience them for the first time. She spent so much time looking forward, that given the chance to go forward or back, she would have learned through trial that looking to the future isn't always the best way to spend the present.