IMO there are several factors on which to base your decision but I think the two most significant are appearance and cost.
Personally I would treat T5 HO and MH as equally efficient. So if you were thinking of 2 x 150W MH on a 4' tank then you are looking at a 6 x 54W 4' T5 setup. Despite a lack of hard data you will see people claiming that T5's are WAY more efficient than MH. I find that hard to believe considering T5 is the same fluorescent technology from T12, T8, and PC only with a better reflector. The heat difference comes about because heat is distributed along the entire length of the T5 lamps vs a much smaller volume in a MH lamp.
Pricing on a 6-lamp 4' T5 is comparable to a dual MH unit. Long term lamp replacement costs may be higher than MH if using aquarium grade lamps. On the plus side you can mix and match spectrums to get the look you want. With MH you are stuck with one bulb type.
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