Still wrapping my head all the way around this one. So it does sound like you are working harder on the heavier bike and that is good. If the time you spend on the bike is the same, its just the distance that changes and you are seeing your hr higher then you are likely getting a better workout.

In the end it doesn't have as much to do with the bike as it does with good monitoring and adjusting your workout.... I can get the same workout with any of my bikes, the only variation will be the distance covered. The gearing is a bit lower on my heavier bike, so I'm probably not getting any more workout in that respect.

If you have a reason to go less distance, but want a harder workout use the heavier bike. For example I can get a good workout on my commute, which is a set distance by using a heavier bike. If I were to use a lighter bike then I'd have to find extra distance to make up the time (my workouts are all set by time) - With the heavier bike I can still do the workout, but since its slower it takes the requisite amount of time. If you are doing a lot of stop and go, then the heavier bike may be the best choice. For me at least, with the lighter bike it takes longer to get up to the amount of effort I need. If its stop and go, I may not be able to get in enough hard efforts on the lighter bike.