ok I've done t5 had a tek fixture, worked fine with SPS for 4-5 months then sps started browning, with my MH I can go over a year with no probs and par readings after a year compared to a new bulb are not that drastic of a difference. yes sps and other high demand coral can be done with t5 just be consistent with bulb changes. changing 4-6 bulbs every six months is expensive. when I can change one mh bulb at $80 a year. and the cheap mh bulbs I'm not sold on or would risk my coral for 60 dollar savings.I run my MH for 13 hours a day with no algae or heat issues my fixture has fans and I have a walmart fan that kicks in at 77 degrees, tank only hits 78 all year.
Gabe had t5s on his show tank and noticed bulbs had to be changed at about 5 months. now with his MH fixture the tank is looking good and bulbs last longer. also talked to other people who have the same prob. now granted these are people who have very light demanding livestock so more bulb changes where needed to stay consistent.
as for heat i would not recommend more than one or two MH under a canopy without some good air movement. but any bulb under a box is going to create heat. if your not going to be running more than 2 mh bulbs electrical use is not a big deal in my opinion.
so in the end make sure you weigh out what your situation is going to be: what coral/livestock do you want, are you going to have a hood, and last how much do you want to spend.
LEDs are cool but i don't think bugs are worked out yet. people always have some kind of problem, controllers or lights going out, lil things really but still down time to fix it.