Heat oven to 350°F. Combine sugar and 3/4 cup butter in large bowl. Beat at medium speed, scraping bowl often, until well mixed. Add eggs and vanilla; continue beating until creamy.
Combine 2-3/4 cups, baking powder, baking soda and salt in another medium bowl. Add to butter mixture alternately with sour cream, mixing well after each addition. Toss together rhubarb, strawberries and remaining 1/4 cup flour in same medium bowl; gently stir into batter. Spread evenly into greased 13x9-inch baking pan.
Combine brown sugar, 1/4 cup flour and cinnamon in small bowl, cut in 1/4 cup butter until mixture resembles coarse crumbs. Sprinkle evenly over batter. Bake for 45 - 55 minutes.