My reasons(though I have actually never had them):
1) To get the same amount of output as MHs you have to have lots of them..Personally just to much space.
2) Again my opinion... Even if you were to get the same amount of wattage the intensity needed to keep an sps coral healthy 24" deep in water is going to be lacking.
3) Single light point. MHs make shimmer light lines in your tank, like the sun does on the ocean floor... Extremely cool looking.
The downside to MHs over T5s is heat.
Again please keep in mind I haven't ever personally had t5s, but my opinion of them is they are just a smaller VHO bulb that allows you pack more of them in a given space. Even if they were exactly the same I would go MH for reason 3 listed above, because T5s will never give you that.