How beneficial is it to swap bikes?
Bike weight and performance is only one factor.  Your average speed and whether or not you have inclines would also factor in, of course. You can have a light bike with skinny wheels and burn as many calories by averaging 18mph, or a heavy bike and burn the same calories at a lower speed, etc...  Coincidentally, this month's Bicycling Magazine has a brief illustration on how much power is saved by tweaking tires, helmet, clothing, etc...