Whats wrong with seting up both and do a dusk-to-dawn effect? At only 2.3wpg, it won't even meet the standard of medium intensity anyway. I have found that my fish are less stressed when the lighting is more gradual and it has a cool effect. Kinda like it's own little world with it's own little sun. Not to mention that your at 24" (20" effective?) and your shorties will be fighting for light.
Everyone else is right. Those bulbs are a bit pricey. I shop for nearly everything online using bizrate, shopping, and ebay to find the best prices.