Losing weight off your person is the best, if you're at all overweight. Besides being healthier, that's high CG weight.
(Good thing I don't care about weight savings - I could use to loose 50 or so.)
As for the bike, how much to you want to spend on a weight loss program? Besides what tacoman already mentioned, there's not a whole lot you can do without racking up some serious expense, and if you can do that, you should just buy a lighter bike.