Metal halide light is close to regular room light or cool fluorescent light (blue spectrum) and is best for vegetative (beginning) growth. While high pressure sodium is in the red end of the spectrum and is best for flowering (later) growth.
You can run
1) either mh or hps light through both stages of growth (good).
2) run a mh light through the veg phase of growth followed by hps light through flowering (very good).
3) run both mh and hps light through all phases of growth (best).
I've found either a single metal halide or high pressure sodium light system works nearly as good as a dual system. So if you only have one or the other don't worry, it will work just fine.
You can't use a standard hps bulb in a mh fixture, but you can use a mh bulb in a hps fixture of the same wattage. I didn't think this was possible till several years ago when someone I trust told me it was possible. So I decided to try using a 400 watt mh bulb in a 400 watt hps fixture.
The plants are growing as well as they did under the mh bulb in a mh fixture. The bulb hasn't had to be replaced and the ballast runs as cool as it does with a hps bulb. When I want to use the hps bulb, I simply remove the mh bulb and replace it with a hps bulb.
So if you can only afford or only want a single fixture, I recommend getting a hps fixture and standard mh and hps bulbs rated for the same wattage as your light fixture. Use the mh bulb for veg growth and the hps for flowering. There are hps bulbs that can be used in a mh fixture and vice-versa. But these are special bulbs and cost about double the price of a standard bulb.
Bear in mind, you can only use a bulb of the wattage you light system was designed for. A 400 watt system only uses 400 watt bulbs. Don't use any other wattage than the one listed on the ballast of your light system. One exception I have seen is a 430 watt bulb designed for a 400 watt system.
There will also be a lamp type specified on you light system (probably on the ballast itself). My 400 watt metal halide setup specifies an M59/H33 lamp type. M59/H33 is an ANSI (American National Standards Institute) ballast rating. M59 is a 400 watt metal halide ballast. H33 is a 400 watt mercury vapor ballast. So my light fixture could run a 400 watt metal halide or 400 watt mercury vapor bulb.
400 watts is about the best size of light for growing hydroponic marijuana in a personal garden. You should be able to grow 4 to 12 plants at a time. 250 watts should be good enough for up to about 2 to 6 plants. Anything larger (the next standard size is 1000 watts) and you are going to draw a lot of electricity. Anything smaller will limit the size of your garden to one or two plants, unless you can supplement it with strong sunlight.
Expect to pay $150.00 to $500.00 for a 100 to 1000 watt lighting system. You can get a 400 watt gym type (high-bay or low-bay) light (hps or mh) for about $150.00. While a 400 watt horticultural quality grow light system will cost $300.00 or more.
I use a 400 watt metal halide gym type light (high bay) on an 11 plant garden, it works fine with 6 to 10 plants. I usually grow 3 to 6 medium size plants at a time. I start the flowering phase at about 12 inches and the plant grows to about 18 inches at harvest time.
If you can afford a lighting system made for horticultural purposes with a good reflector and ballast, get it. It will last longer, allow the light to cover a larger area (more plants), and increase the amount of light individual plants get.
You can also find some high-end dual (hps and mh) fixtures. These dual systems can provide both mh and hps light through both growth phases. You might want to consider one of these, if you haven't already bought your lighting source yet. If you can't afford better, a high-bay or low-bay type light will work fine. I use one.
The best source I have found for buying lighting fixtures, bulbs, and other hydroponic equipment at very good prices is eBay. There is lots of competition and selection.
