This seems like a fun conversation
I think that it sort of depends on what your goals are. For me, when I exercise on a bicycle it is more a function of how hard I pedal than how fast I go. I'm a bit of an introvert and so I don't really have any friends to ride with and so I'm not going on any group rides. I've also never been fast at anything (except in the parallel universe in my head where I'm in charge of everything). So in my situation I feel that as long as I'm pedaling hard enough to get my heart rate up, then I'm getting the aerobic exercise benefit from cycling. While my goal isn't to be the fastest person on a bike, I don't want to be slow either. So if the question is whether or not a heavier bike is a better work out, I think that it just depends on how hard you pedal. If I were a competitive cyclist, I know that I would prefer to train on the bike that I would be racing with so that I would be accustomed to all of the nuances of that bike such as handling, gears, braking, etc.