Your body burns fat at a fairly constant rate no matter how hard you exert yourself. Or at least that is what I've read from several places. So it seems to me that long rides done at a moderate aerobic level will go further toward weight loss.
My own anecdotal experience finds that short hard rides just have me eating back the Calories I burned. Long rides seem to get the weight off me but I don't devoted enough time to longer 50 mile plus rides lately. So my weights not quite were I want it.