Hello Davephan,
It all depends on your spacing, location, and how much your growing. HID lighting will give off more heat so they need to be farther away from the plants which isn't a problem since the luman drop off rate is much lower with HID. However if your growing in stacked rows like shelves the extra height needed may be an issue. Also With a 4 foot florescent light you get even lighting along the the 4 foot length. Where if you used one Hid bulb in the middle, the plants in the middle will get a much higher light intensity, and the ones around the edges would get lower light intensity, so the growth rate wont be even. You also may have an issue with light shading. That is the plants closest to the light blocking/and shading light from the plants farther away. With multiple
bulbs you can reduce light shading problems, as well as get even lighting over the whole coverage area.
But if your looking for more coverage area, HID lighting certainly is the best option. It all depends on how much coverage area you need. That all comes down to planning. Deciding on how many plants you need to grow, how much coverage area you need, where you will be growing, how you will be growing. Even controlling temperatures all come into play.
But ultimately you could easily get more coverage area out of using multiple low wattage HID
lights than florescent lighting. As an example 4 T5 lights (216 watts) would easily cover a 2x4 foot space (8 sq feet), but 2, 100 watt HID lights spaced evenly could cover a 6 x 3 foot space and could more than double that coverage area (18 sq feet) using about the same wattage.
P.S.
Do the $60 lights your talking about come with the light fixture and
ballast? I'm aware of a 100 watt HID light that comes with the fixture and ballets, but runs about $114.
Mini Sunburst MH w/Lamp, 100W