Cook shells according to package directions; rinse under cold water, drain and pat dry. Meanwhile, heat oil in a large saucepan over medium heat. Add onion and chorizo and cook until browned, about 5 minutes. Drain all but 1 1/2 teaspoons drippings and return pan to stove. Add tomatoes and sugar and season with salt and pepper to taste; simmer until sauce is slightly thickened, about 10 minutes. Set aside.
Preheat the oven to 375 and coat a 9-by-9inch baking dish with cooking spray.
Combine spinach, cheeses, egg whites and 1/4 teaspoon each salt and pepper in a large bowl. Spoon half the tomato sauce into prepared dish. Stuff each shell with 1 heaping tablespoon filling and arrange stuffed shells in baking dish. Spoon remaining sauce over shells and cover with aluminum foil.
Bake until pasta is cooked through and sauce begins to bubble, about 30 minutes. Let rest 5 to 10 minutes before serving.
