For soup, you're going to want to seek out something that's made for eating. The kind made for carving are grown specifically for looks and size; inside, those bad boys are watery and flavorless. Edible pumpkins are mostly winter squashes so you can mix and match here. Hubbard or butternut squash would be excellent in this soup. The softer the flesh, the more watery it will be, so keep that in mind when choosing. As far as pumpkins go, anything that's sold for pies will also make a terrific soup--not too watery and with a lovely, sweet flavor. Sugar (also called sweet) pumpkins, Long Island Cheese pumpkins (so-called because they look like a wheel of cheese), and grey Kakai pumpkins are all excellent choices.