Don't worry about cadence for weight loss. Worry about biking more overall, biking further and harder, and then most importantly - eating right.
Calories in, calories out. That's weight loss in a nut shell.
The cycling will keep you healthy and fit, and I've definitely used it to my benefit to lose a LOT of weight in the past year. (65+ pounds lost so far, still going. 22 BMI now, started at 32).
That said, I'm a big proponent of high cadence. Easier on my knees and legs, and lets me put most of the strain on my heart, which has gotten stronger and longer lasting than my legs.