Oh, that is such a great question!
If you only had to do one, choose strength training as muscle burns fat. Ideally, though, you would do both strength training and cardio. Be sure that both types of exercises are included, as well as flexibility, to ensure a well-rounded weight loss program.
My suggestion is that cardio be done 4-5 days per week, strength training 2-3 days per week, and flexibility included in your cool down after each session.