Sun Drop is better than either of them by a wide margin. But I only like Sun Drop in the cans. For whatever reason, it has a flat taste from plastic bottles and just isn't the same. Sun Drop draws heavily on the orange side for its citrus flavor and is delicious. In comparison, Mountain Dew tastes like cirtrus flavored diesel fuel. Mello Yello is kinda just bleh.
I also prefer RC over Coke and Pepsi and don't understand why it doesn't have a bigger following. It blows the other leading colas out of the water in terms of taste and I love the heavy carbonation. When twisting off the cap on a two liter of RC, the fizz is so strong it nearly sends the cap into outer space!