Well, they make several different makes for the t-5 lights and I think a 4 lamp fixture would be plenty. If your tank can fit the 6 bulb fixture than go for it but the 4 would be fine.
T-5 uses less electricity but could cost more to replace bulbs than mh, depending on how many bulbs you have and how often you replaced them on the t05 fixture. t-5 lamps are about 25-30 bucks a bulb but they last over a year. Metal halide 150 watt hqi lamps range about 50-60 dollars a bulb up to around a 100. Metal halide bulbs need to be replaced about every year or so. I don't like pc because they aren't very intense, and I think they need to be replaced pretty frequently. definately more than once a year.