Our designer and our builder both recommended a flat surface for our 6x9foot center island with 6 burner stove plus grill. I am glad that we chose the raised bar that is my stand-up desk at which I am typing this, as well as a nice place to sit and talk with the cook -- the island includes two ample working surfaces to the side of the stove, plus one raised area, adjacent (an L shaped bar). The sink is opposite the stove, overlooking a back yard. The first reason I wanted a higher bar is that the open plan meant that my kitchen would be visible from my large foyer, both the stove and the sink. In addition both would be visible from my large dining room by guests. My house is about average for my area -- 4-5,000 square feet -- older ones are smaller, newer ones are larger. It was not built with an open plan, but the dining room is larger than the living room. For parties, raised bar is the center of the action -- it serves as extra seating before dinner, as well as a place to set up glasses for drinks, and then to serve food. I don't have a separate bar area, and consolidated a second sink near the ice maker/refrigerator and the end of the 42" bar. The bar also keeps the helpers out of the cook's way, channeling traffic around the L shaped kitchen.
The cost was a little less, I believe, as it is hard to find a 9 foot slab; even so, our kitchen used 4 slabs of granite. The higher level of the bar had an unexpected benefit -- it also serves as a backsplash for the stove. One consideration is whether you will want a 30" high surface for rolling out dough, or access by small children or seated users to help in the kitchen. We use a nearby table, but if you don't have such an opportunity, you may wish to have a dual level counter, but one part LOWER rather than higher. My neighbor with the same floor plan chose this arrangement, even after seeing mine, because her family liked it that way. So, both of us liked our choices, and I have no regrets with the higher 14" wide bar that provides seating for six, or a buffet for 26. It's a personal decision based on your floor plan and how you are going to use your kitchen. We even served dinner there once to another couple, although I would probably not do that again. However, if guests are taking turns preparing different courses, it could work. I have also been advised that the kitchen is an excellent arrangement for small cooking classes. Students are seated just a few inches higher and have a better view of the chef at the stove. Good luck!

Remember that you can always add a higher level with another or material, but it is hard to go from a raised bar to a flat surface; if in doubt, choose a flat surface.