The answer is easy. You get more lumens per watt with HPS.

This is a quote from Roughrider's stickie. It should help you figure out how much light you need.

"2000 lumens sq. ft./21500 lumens sq. m. = Absolute minimum for growth. You won't get much from this, especially after the plant has grown a bit. Not really enough to flower well.

3000 lumens sq. ft./32250 lumens sq. m. = Pretty Good growth. Enough light for the entire light cycle, although your yields may be lower.

4000 lumens sq. ft./43000 lumens sq. m. = Very good growth. Once you pass around 3500, growth rate and ability goes up fast."

Another quote from the same thread will show you the difference between CFL and HPS.

"But HPS is more efficient. A typical 250 watt HPS bulb/unit will produce about 27,000 lumens. I've seen people use a 250w in a 3' x 3' room and get good results. That's 9 sq. ft. which = 3000 lumens a sq. ft. (Really, a 250w HPS is better in a smaller area.) to give you an idea of the difference in efficiency of CFL vs. HPS, think of this.

23w CFL = 1600 lumens = 69.6 lumens/watt
30w CFL = 2000 lumens = 66.7 lumens/watt
40w CFL = 2600 lumens = 66.3 lumens/watt

compared to

150w HPS = 14000 lumens = 93.3 lumens/watt
250w HPS = 28000 lumens = 112 lumens/watt
400w HPS = 50000 lumens = 125 lumens/watt
600w HPS = 90000 lumens = 150 lumens/watt

So you can see that HPS is more efficient than CFL...and as you get into bigger HPS bulbs, it becomes a lot more efficient. There's also fewer hassles with multiple cords and saved money on your energy bill. If you've got a big area and/or you can deal with the heat and ventilation, HPS is the way to go in flowering. Still, I'm a believer in small HPS lights and combo HPS/CFL grows...if you've got a 2' x 2' room, you can use a 150w HPS and 4 23w CFLs from Wal-Mart and get a terrific grow with very few heat issues. "

Hope this gives you the answers you're looking for.

For what it's worth, I am currently growing with CFL's. In the future I will continue to veg with CFL's but will most likely upgrade to HPS for flowering as it is said to give more compact and heavier buds. The HPS is also said to penetrate the canopy of the plants better so that you get more out of lower buds.
McToker Reviewed by McToker on . Confused about lighting & info on fans PLEASE I have a 6ft x 4 x 6ft high grow room (it's actually 8ft wide but I want to partition 2ft off to try SOG). So far I have 2 x 250W CFLs (red spectrum) in reflectors. Now, I know this is nowhere near nough light to flower 8 x 4ft high plants so I need to buy some more lights. I went for CFL because a) they run cooler than HPS b) according to the makers blurb, they are more efficient as their light is 100% PAR, where as HPS waste alot of the power they use as heat and light the plants can't use Rating: 5