This is an article I dugg up. Not to in depth on comparison but a decent summarization .
HID vs. Fluorescent
The main difference is that fluorescent’s create light by passing electricity through a gas vapor under low pressure and HID creates light by passing electricity through a gas vapor under high pressure. HID lights are much brighter and while initially more expensive, are more cost efficient to operate and will grow a much nicer crop. Therefore, they are the light of choice for most indoor growers.
Fluorescent
Fluorescents come in a wide variety of shapes and sizes. There are compacts, twist bulbs and circle bulbs. They all work the same way. They have a starter and ballast which help provide a steady and regulated amount of electricity to the light. Before HID lights were available indoor growers used fluorescent lights. While fluorescents can be used for an entire grow from seedling to harvest, it is generally not advised to use them in this way. To grow effectively with fluorescent’s, think small. These lights are better suited for starting seedlings, rooting clones and small plants, maintaining moms for cloning purposes, and providing sidelight for the undergrowth. These situations do not require a high intensity of light to run their course. The light emitted by fluorescents is gentler and more diffuse, it doesn’t release a lot of heat, and doesn’t make the plant work as hard. For this reason they can be kept closer to the plant. One or two inches is sufficient, but this also means that the lights must be adjusted almost daily to accommodate growth which can be troublesome.
High Intensity Discharge
There are basically three types of HID lights. Mercury vapor (MV), metal halide (MH), and high pressure sodium (HPS). These lights also require a starter and a ballast.
Mercury Vapor
Mercury vapor is the type of lights that were used for streetlights many years ago. Not very good for growing because it doesn’t provide enough of the right kind of light spectrum. While they do provide a littleof the blue spectrum, MV also produces too much heat to get very close to a plant, and are very inefficient to operate.
Metal Halide
The metal halide is a very good source of the white/blue spectrum of light that is ideal for vegetative growth. Many growers use MH during the vegetative phase. MH is bright and cost efficient to operate, but not as efficient as HPS lights. Most commonly used sizes are 400 watt and 1,000 watt. Works best when used in combination with HPS lights.
High Pressure Sodium
The best light available today for growing marijuana. High pressure sodium lights are very bright and very efficient. This light has a red/orange spectrum that is ideal for the flowering phase. With enough of these kind of lights you too can grow those centerfold buds. HPS comes in a wide variety of wattages from 70 watts up to 1,000 watts.
Lighting Forumlas
To correctly determine the best lighting for your space there are several things that you have to know. At this point, a couple of definitions are in order. The amount of light emitted by one candle that falls on one square foot of surface one foot away is called a lumen (lm). The amount of electricity flowing through a wire is measured in watts. Watt (W) hours measure the amount of watts used in one hour. A kilowatt/hour (kWh) is 1,000 watt/hours (Wh).
Operating Costs
Find your kWh charge on your electric bill. Assume you have a 1,000 W light and your kWh charge is $.05/hour. A kilowatt (kW) equals 1,000 W, therefore it will cost you $0.05 cents per hour to run that light. Here’s another example. Say you have a 400 W light and your kWh charge is $0.03. Since 400 W is not a kilowatt, you must divide 400 by 1,000 (0.4 kW) and multiply that by the kWh rate from your electricity bill (0.03 kWh), leaving you with a cost of $0.012 cents per hour.
Lumens Per Square Foot
To determine how many lumens per square foot you have, find out the square footage of your space by multiplying the width and depth. Divide the lumens available by your square footage. This will give you lumens per square foot. For example, say your space is 3 feet deep by 4 feet wide, for a total of 12 square feet. The total lumens available from your light(s) is 45,000 lumens, which means you have 3,750 lumens per square foot.
How Much Light Do I Need?
Technology has advanced so much in the last 15 years that we are constantly refining the process and updating what we know works best for growing. Current theory holds that the minimum amount of lighting needed to sustain growth is around 2,000 lumens per square foot. Mid range is around 5,000 lumens per square foot. Optimal is 7,000 to 7,500 or higher lumens per square foot. How Many Watts Do I Need? The general rule of thumb for providing light for an area is a minimum of 30 W per square foot. 50 W per square foot is optimal. You can determine the proper lighting for your area by using this formula: Watts x Square Feet For example, say you have an area of 10 square feet and a 30 W. That would mean you have a 300 W per square feet minimum. Also, remember that fluorescent’s are weaker and emit less light than an HID. This means you will need 5 times the amount of wattage to equal the output of an HID. So, 30 W of HID would equal 150 W of fluorescent’s. This is why it is advised to provide a minimum of 30 W per square foot for HID lights and a minimum of 150 W per square foot for fluorescent’s. This is all important because the light intensity will directly affect the quality and yield of your crop. If you have less than optimal lighting your yield and potency will be reduced and buds will not develop as dense. This point can not be stressed enough. You must have the right amount of light for your space to grow high quality bud.
Can I Have Too Much Light?
The basic answer is no. According to the law of diminishing returns, you could theoretically reach a point when your plants just couldn’t absorb any more light, but it would be impossible to have that many lights in your space. Heat from the lights would become a problem long before you ever reached that point. So use as many lights as you want, just control the heat. Experimentation is the only sure method to determine the best solution for each plant. If plants are not receiving enough light, they begin to grow tall and spindly as if stretching for the light and foliage becomes pale green. Or, if they need to be moved closer to the light, or given a longer light exposure period. Too much light may lead to bleaching of leaves and flowers, browning and shriveling. Leaves would become overly compact and curl under at the edges.
Photoperiod
Your plants should be started and taken through vegetative growth with a 24/7 or 18/6 light regimen. The reason for an 18/6 regimen is to give the plants a short dark period to breathe and to reduce your electric bill a little bit. Most plants thrive with at least 16 hours of light a day. Adjustments should be made according to individual plant requirements. For flowering 12/12 is the norm. Again, adjustments may need to be made. A minimum of 12 hours of darkness is required to trigger the flowering process.
Lighting No-No’s
Don’t burn your plants by getting them to close to the light(s). Fluorescent’s do not put out much heat and can be as close as one or two inches. HID lights get much hotter and will need to be farther away. A good test is to put your hand between the light and the plant. If your hand gets too hot for comfort, the light is too close. There are some common lights that may induce a seed to come up, but are worthless for growing purposes. These lights include; Any incandescent (regular) light bulb, halogen lights, black lights and heat lamps. Don’t waste your time trying to grow with these lights, you will only be disappointed.
CFLs can work great, if tweaked correctly, but will definitely not "dominate" HIDs. HIDs are more efficient, watt for watt, which is the main reason they're so popular with growers.
