Most recommend using one MH lamp per 2 feet of tank. Mr. Salty is using two for a six foot tank, which is fine, as long as you put the higher light animals closer to the lamps. VHO lamps need to be replaced once per year when using electronic ballasts like the Icecap (six months when using a magnetic ballast), MH can last up to 18 months. If you are interested in keeping high light corals like some SPS, you could consider using higher wattage lights like 250 or 400 watt MH. When you get into high wattage like that, MH is more cost effective than VHO, and it can definately penetrate deeper into the water. For tanks deeper than 24 inches, MH will be THE choice. The more bulbs you have to replace, the more expensive. Just add it up. The ballasts are similar in cost. If you have a bunch of VHO tubes, you may find it cheaper to use MH. If you choose 10,000 K lamps, you can use only the MH and not use any supplemental lighting.
Sonny