Metal Halide (MH) lamps are usually available in 150W, 250W and 400W although less popular configuration such as 175W is also available. If your tank is 2 feet in height or more in height and intend to keep SPS, you need 250W or 400W in order for the light to penetrate all the way to the sandbed. 150W lights may only allow you to keep SPS to about 1 foot depth.
Bulbs also comes in 2 types of end fixing, Double Ended (DE) which mean fixed at both end and Single Ended (SE). Generally DE fires brighter than SE bulbs but they have no UV protective coat and therefore required a glass pannel to eliminate the UV. There are also 2 types of ballast, magnetic and electronic ballasts. The magnetic ballast tend to drive the bulbs harder thus a brighter bulb but will appear more yellow.
T5 comes in the following configurations, 2ft (24W), 3ft (39W), 4ft(54W) and 5ft(80W). Assuming a tank height of 2 feet, you may require at least 4ft(54W) tol allow you to keep SPS at the bottom of the tank. There are other flourescent tubes include the Very High Output (VHO) tubes. These are tubes that has a higher output per tube as compared to the T5.
Lighting is one of the most important pieces of equipment in a reef aquarium. The proper lighting intensity is required for your corals or invertebrates to survive. Similar to plants, corals and many invertebrates require light for photosynthesis to generate energy. If your corals and invertebrates do not received enough light for photosynthesis, they will not survive.
I hope these help.