I'm using T5. Right now I'm using a Tek 8x54w. The only thing I'll be switching is the fixture as soon as I can get an ATI fixture.
I'm looking at the ATI Sunpower 8x54 watt fixture right now. My tank is 55"x24"x26". I haven't read nearly anything bad about this unit, and the fact that the most beautiful SPS tank I've ever seen is lit by a powermodule, I figure its a good starting point.
The June 2010 Tank of the Month built by Krzysztof Tryc (lunar) is absolutely inspirational.
I have spent an incredible amount of time getting my system balanced and consistent. I dose, have an ATO, test regularly, and keep a very strict maintenance schedule.
I have made it an absolute mission to keep my parameters within the optimal range. I run a DIY Cree LED setup from RapidLED.com and I'm not overly happy with my results. Some of my corals look phenomenal, however there are downsides to the LED. I find the light to be very directional, and find that it casts shadows more so than any halide or T-5 I've used in the past.
I also find that the shimmer is so erratic that its not natural like with metal halide. I realize that this is due to 72 sources of light as opposed to lets say 3 with halide pendants. (and I realize that this point is almost irrelevant since T-5 has no shimmer at all and thats the direction I'm considering going)
Above all I realize the leaps and bounds that LED's have made in a very short time and realize that its too early to commit to a very expensive LED setup.
When I first started this new tank 18 months ago, the AI Sol was the biggest breakthrough in LED's ... then the Radion showed its face ... improvements are made on them so fast that most people haven't paid their credit card bill off before the next best thing gets released ... making models before them completely obsolete.
The discovery of varied spectrum and uv ... is inspiring new units every single day it seems. Yellow, Green, Cyan, Red ... colors that weren't even mentioned when the first Royal Blue / Cool White fixtures were released.
LED's are evolving like Laptops did a decade ago ... I guess I just want to stick with tried and true until the advances slow down a little. I don't want to spend 3 grand and have a unit thats missing key features 10 months from now, when the experts in the field of lighting discover another key element thats critical to long term success.