i have halides now and i miss my t5s bad. I love the shimmer no doubt,..but my growth has considerably slowed down. Think about it interms of lighting :
T5 are more of an area light, metal halides (depending quite a bit on the reflector) are spotlights. I have a huge purple monti cap that is going white where it is ahding itself, because the light of the mh is so directional,..while the t5 is pretty spread out. Now compare tall that to say the sun,...the sun is a point light just like an mh,..but its so far away that for all intents and purposes its an are light.
So if you go wit MH make sure to get a GOOD reflector and T5s for supplementation (aesthetic purposes),..if you T5 then you will not get shimmer and no shadows (which create intresting patterns and a sense of depth) but you will get great color and amazing growth
this is my experience,..i would go with both. An mh/t5 combo