If you do the same exact activities day in and day out, then the amount of muscle you have should adjust according to your weight. So if you lose fat weight, you're bound to lose a bit of muscle along with it. This being the case, losing 5 lbs off your bike is actually probably slightly more advantageous than losing it off your body.

The reality is that nobody loses exactly 5 lbs one way or the other, and if you're losing weight, you're likely to be exercising more, etc. If you want to go faster, get a better bike AND lose body weight rather than one over the other.