Depends on your budget, what corals you'd like to keep, and what you find pleasing to your eye. Is your fixture a 48" ? T5 are the cheapest on the front end, but replacing bulbs every 8 months, and then one day upgrading may not be cost savy. Metal halides are expensive up front, but are pleasing to the eye and you can keep a wide range of corals. LEDs are pretty expensive now, but very reliable, cool operating temp, wide range of colors and ability to keep corals.