If it were me I would get the built in type. They often come with a cover to convert from a grill to a flat top which is a feature I like. I have a double sided Magnalite grill/flat top that sits atop 2 burners on the stove. It works fairly well but there are definite hot and cool spots which can be annoying if you are doing pancakes or such. The grill side gives nice grill markings but doesn't do anything to enhance the flavor like grilling on an open flame does.
I suppose the down side of the buit in model is the cleaning, but for me it would be worth it. You also need a good hood vent with the built in type but again, the benefits out weigh the down sides by a long shot.