Ask the manufacturer for the spectrum. That's the only way to tell which bulb is 'better' for growing plants. Subjective brightness doesn't really tell you anything. Remember that our eyes are more sensitive to light emitted in the green area of the spectrum, so any bulb with a large spike in the greens is going to appear brighter to us than a similar bulb without a spike there. As an example, a cool white bulb won't grow plants worth a hill of beans despite appearing very bright to our eyes (owing to the heavy emphasis on the green part of the spectrum) compared to a ZooMed Flora*Sun, even though the latter appears dimmer. However, keep in mind that when you're talking about a compact fluorescent, the margin of benefit between one bulb and the other probably is not all that significant at the wattage you're dealing with. Unless the spectra are markedly different, go with whichever one you feel shows off your tank's colors the best.
Mike, I'll give you another example:
Let's say you wanted a bulb that glowed green, say at 3000K (making up a number). You're given two bulbs. The first one has just one phosphor that radiates (i.e., has a spike) in the green area of the spectrum. Boom! You've got a green bulb radiating at 3000K, right? But the second bulb has two phosphors, one that radiates in the yellow area of the spectrum and one that radiates in the blue area of the spectrum. Yellow + blue = green. Boom! You've got another 3000K bulb! How do you determine the spectra when both bulbs appear the same? You can't: That's the point I've been trying to make about predicting the spectrum from the color temperature rating. Of course, real-world physics doesn't quite work out as cleanly as it does in this hypothetical example, but hopefully you get the picture.