Working out with weights slows down loss of lbs, but speeds up the fat loss. So I do both. The more muscle you have, the more cals you will burn, and naturally speed up your metabolism.
I am not trying to bulk up either. I do lots of sets and reps with low wieght.
Doing cardio only and diet you will deff drop 10-15lbs real fast. Diet is where its at. with me, it was not that I had to eat diet food all the time, I just stop eating bad food all the time. Retrospect it was easy. I eat more now than I did back then.