I'd pick the Mushroom Kingdom any day. The Star Carnival wasn't much of an important location in the Mario universe. The Kingdom is one of my favorite locations in the Mushroom World, that and Sarasaland. The Carnival was okay though.
The only thing that intrigues me about the Star Carnival is the number of balloons in the air
Mushroom Kingdom. The Mushroom Kingdom is everywhere, mentioned everywhere. The Star Carnival? Only in Mario Party 8, and nothing more. Star Carnival is pretty much a one-time place.