A roast is a roast is a roast.
Roasting is a method of cooking that involves the aplication of a lot of dry heat to a peice of meat. You can obtain a great roast from all the above the cooking methods as long as you take into account the various ways they work.
A rotisserie offers all around even heat and will remain juicy although you can lose some juice from the skewer through the center, but may not cook the roast as perfectly as you lack good tempature control.
The oven is a great way to roast meat as long as you use the two tempature method. That means you start high and finish low or vice versa. I tend to side with Alton Brown on this and start low (normally 200 but you can adjust according to time requirements) until my roast is ~25-to-20 degrees of medium rare, pull it out and bring the oven to 450, put back in and remove to rest at 15 degrees short of desired doneness. You also have to make sure that you raise the roast off the bottom of the pan because if you don't you will end up braising the roast in it's own juices.
Your grill can work as long as you keep the lid closed and the roast from directly over the coals, but it is going to be the method that needs the most constant attention and will be easy to end up with a overdone outside and a underdone interior.
There are two general approaches to making chicken juicy in the oven. The first is to cook a short time at a high temperature. For example, Barbara Kafka's recipe for roast chicken
calls for cooking the chicken at 500 degrees F for less than an hour.
The second option is to cook at a low temperature for a very long time. This recipe calls for cooking for an hour at 250 degrees F, with a high heat sear at the beginning and end of the time. Even more extreme is this recipe, which cooks at 140 degrees F for 4-6 hours. However, low heat will not give the yummy crisp skin.
Neither of these requires flipping the chicken.
However, if you really want the crispiness of the skin, flipping is the way to go. Two recipes from Cooks Illustrated (one and two) both call for high heat and a couple of flips. (As does Barbara Kafka's recipe for cut-up chicken, which I make all the time. Season the chicken, and put in a 500 degree F oven for 10 minutes, flip, 10 more minutes, flip, and 10 or more minutes or until the skin is crispy.)
If you want to go with the classics, Julia Child's recipe for roast chicken from also calls for turning the chicken onto different sides. She also bastes frequently, although the above recipes don't call for it.
Best Answer
Dry and even heat. There is still a difference, even with the same cut of meat and using a roasting rack to avoid roasting the cut in its juices: meat on a rotating skewer will be exposed to uniform heat. If you set the temperature of a home oven to 350°F, the top of the roast will be exposed to that temperature, but the bottom, in line of sight to the heat source, will be much hotter.
Commercial rotisserie ovens also cook the meat by convection and use the heating elements to create the outside crust. As the meat roasts, it becomes darker and absorbs more of the radiant heat from the roasting coils. So the outside ends up getting overcooked unless the roasting coils are carefully controlled, which is kind of hard to do. In a convection oven the temperature is constant around the roast.