Would there be a difference between boiling whole black peppercorns or fine ground black pepper in a stew?
Well, I've never had whole black peppercorns dissolve.  In very long cooking, and depending on the variety and age of the peppercorn, they can soften somewhat.  But when adding whole peppercorns to a dish, I either plan to remove them after cooking or be prepared to bite into a serious bit of pepper every now and then.  To avoid this problem, I generally at least "crack" or crush the peppercorns a bit if I intend them to be in the final dish.  (This aids in softening and seems to allow the flavor to blend a bit more, though large bits can still be very peppery.)

Black pepper tends to lose a lot of subtleties of flavor quickly once it is ground, which is why pepper grinders are so common in restaurants (and at home).  There's no advantage of using pre-ground pepper in a soup or stew, but I definitely tend to grind it myself right before adding to avoid the large chunks of peppercorns.  (Different varieties of peppercorns can vary a lot terms of potency, especially when they are fresh, so perhaps yours are more mild than the ones I generally use and thus less noticeable when in larger chunks.)

Some people buy pre-ground pepper and find it convenient, though.  My one caution, if using pre-ground pepper and you desire a very peppery dish, is to not overdo it.  Whereas whole peppercorns will gradually release subtleties of flavor, pre-ground pepper can change somewhat quickly from barely noticeable to a burst of strong pungency.