I'll agree with an earlier post in the thread. Trash pulls are hectic and often stressful, boss fights are relaxing (most the time). I imagine as a healer it would be opposite of that, with trash being easier (assuming the tank is holding aggro), and the boss being more stressful on high movement fights... Having played both in various MMOs, I would say both are about equal in terms of pressure/difficulty. They just get the pressure in different ways.