T5's actually put off the same heat as MH watt for watt. It just doesn't seem like it because the heat is spread out so much more. T5's do put out more par per watt, so you can run less wattage (less heat) and get similar light levels as MH. Since your running less wattage they are cheaper to run and the bulbs do last longer if they are not overdriven. I think I read 18 months.
If your going for pure horsepower, the T5's cannot match a 400W MH bulb. 2 of those will put out a lot more power than the number of T5's that you can physically fit over the same space.
My fixture (2x250 14K + 2x54 actinic) will probably have more par than a 8 bulb T5HO fixture using a mix of bulbs but it will also use a good bit more electricity. (608W vs 432W). However, depending on what your keeping you may not need the extra par which you get from a fixture like mine.
Also, tank size and shape plays a big role in which is better. If your trying to light a 20" or 24" W tank the MH rules because you can do it with a single bulb. T5's for such a small tank is very expensive. On a 36" W tank I think the T5's are better because you most likely need 2 MH for that width. On a 48"W tank, I think it depends on what you want to keep.