Combine buttermilk (or yogurt), 3 tablespoons oil and vanilla in a small bowl. Whisk the remaining 1/2 cup sugar (or 1/4 cup Splenda), all-purpose flour, 1 cup whole-wheat flour, baking powder, baking soda, 1/2 teaspoon salt and 1 teaspoon cinnamon in a large bowl. Make a well in the center of the dry ingredients; add the wet ingredients and stir with a rubber spatula until just combined. Fold in apples, then the beaten whites. Spoon the batter into the prepared pans, smoothing the tops. Sprinkle with the reserved streusel. Bake the loaves until lightly browned and a skewer inserted in the center comes out clean, 35 to 45 minutes. Cool in the pans on a wire rack for 10 minutes. Remove from the pans and finish cooling on the rack.