My advice to prospective PhD students is to follow your passion and pick a topic that interests you — don't do a PhD topic that you hate, but you think will be lucrative. Because the big picture is that it is the fundamentals learned and problem solving skills gained from your PhD that will open the real career doors. Topics come in and out of fashion — it is the investment in yourself and the person you become through your PhD experience that really matters in the end. Of course, if you happen to love a topic that turns out lucrative then great — but this is hard to predict.
Introductory profile at The University of Adelaide