Mainly the price... as you've noticed. Some of the special "plant" lights are not rated daylight. E.g. one of Dennerle's ones is actually casts an orangy light, hence making the plants look wierd. Some have slightly higher wattage then the equivalent length normal light.
Most hobbyist have found that the price is not worth the gains. Any ordinary FL (like Osram/Philips) with good colour spectrum and rated "daylight" are good enough.
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