howdy! i heard a call for me so I came running...
yup I ran a 55 gallon with 4 54w t5's on it for about a year. It was my first sps set up and as far as I can tell stuff grew like mad and my color was good as long as my nutrients were in check.
I know alot of the reefes in this hobby swear by MH, but I haven't fired up a tank with them yet so I can't speak on them, but I will say I think the idea that you can't have an sps tank with T5's is bull.
I found the lighting to be too intense for some of my lower light corals like brains and things. I had some open brains bleach out, a closed brain frag that keeps hanging on, but I have bleached a few times, acans that need shading as well as a few favias. I had several acros, digi's, passamocora, birdsnest, monti's, poccilipora, and stylophora all which grew to verying degrees.
Make sure you make shaded areas in your rock work so that you can keep lower light things with out bleaching them.
T5's have a very even spread of light from end to end on their reflectors were MH don't. So you will need to provide shady spots on your own.
All that to say I woudl certainly go for t5's. I wish I could fit them under the hood of the corner tank that I am in the process of building, or I would. But I can tell you I will be running them on my
frag tank. Love them can't say enough positive about 'em.
HTH