First of all thanks for the book suggestion.
Actually I'm very confident that exercising in the gym or running is much, much more stressful for the heart than freacking out for an exam. Keeping a running rythm for 40 minutes at your maximum capacity is extremely painful and the heart is working much harder than when one has a mental stress. In fact I do not know if it's possible for the heart to work even harder without breaking itself. And it's well known that exercising a few times a week is good for the heart.
I am guessing the difference really comes to what you mentioned: the hormonal cascades. I wonder which hormones that pop ups when we're stressed are "bad" when applied for a long time and repeatedly over days.