Uh oh, now you opened the can of worms. The good ole T5 vs MH debate which has been beaten to death. I'm a die hard MH fan having had tanks run solely on MH and solely on T5s. Imo, you can't beat MH especially when growing SPS corals. You will find plenty of people using T5s on SPS tanks, but let's just say I know quite a few people that have changed right back to the good ole MH. There's a reason for that.
My vote goes for 3x250w halides with 2x48w T5s for supplementals. I have a 4' long, 24" deep tank lit by 2x250w DE halides, and it is very bright...many of my SPS will not tolerate being close to the halides. I keep Milles on the sand, and birdnests and deep water Acros off to the sides away from the direct light. I have a few LPS in there, and they are all shaded by the rocks or they will bleach from too much light.
I use electronic ballasts. You will get more output with tar ballasts, but you will also get substantially more heat as well they will draw more than 250w anyway. HQI ballasts will draw even more, sometimes up to 330w off a "250w" ballast! Personally, I have no need to draw more energy, my bill is high enough, and my lights are definitely bright enough.