It is the same bulb and ballast but the 30" spreads the light into the tank better because the reflector is full length. On the 24", the solid ends will keep the light more concentrated in the middle but will create more dark areas in the top left and right of the aquarium where the light doesn't cover. Many light mounts only attach to the sides of the aquarium top so if you are planning on having them on legs, you might want to think about it. Also, I think the 30" looks better on the aquarium and it dissipates heat better because there is more surface area. I had thought about getting the 24" at first myself, but when I considered these factors and the price difference of only $9 at my LFS, I went with the 30". To each his own I guess.