Saying the bulb has a spectrum of 4100k and that this is not good for growing isn't quite accurate. This measure of light color assumes a continuous spectrum, and is designed to measure how the light looks to the human eye, not to a photosynthesizing plant. Any HID bulb does not release light on a continuous spectrum anyway, and giving it a color number is just an average of all the wavelengths of light it produces. The horticulture bulbs are a higher number because they release both the normal HPS spectrum, and also some blue light that helps with growth. And plants do need light at both ends of the spectrum.

So I'd get the HTG Supply bulb. Actually, I did use one already. It burnt out after a day, but they replaced it free of charge and the replacement has worked perfectly.