Okay so simply put the 7,200K bulbs are closer to 'daylight'. Really the color temp. of natural sunlight is variable, but is generally considered to be between 5,000-6,000K.
The 3,000K bulbs are 'warm' light with more red/orange and generally considered to be more conducive to flowering.
Either can be used to grow or bloom plants, and some prefer to use the full spectrum or mix bulbs. Lumens itself isn't a good measure of how effective a bulb is. Plants don't exactly see/use light like humans do, and certain spectrums of light are more or less usable to the plant. More important would be how much of the lumens produced are usable by the plant, which portion is photosynthetically active radiation or PAR. Figures are available for some lamps but this is getting ahead of yourself anyways.
HID lamps produce less light over time. They operate at peak potential (maybe) for three months and then they decline in output. So, more importantly you can buy what you can afford, or try a higher quality lamp with good reviews. I suppose if this is a first grow, starting cheap doesn't hurt. Don't worry so much about spectrum, lumens, etc. though. You could just go with the 7,000K bulb for the duration of the grow.
