I would look around at what is avialable if buying a halide T-5 lighting system. The most common brand is the Outer Orbit light which has HQI halides and T-5 Actinic lighting, however there are some problems with the T-5 ballasts going out fairly quickly as they are mounted in the reflector hoods where heat is probably an issue. They would be a great lighting system if the ballasts were removed from the fixture and kept remote as are the halide ballasts in the 25O Watt HQI set ups. I believe their 150 Watt HQI combo lighting even has electronic ballasts for he HQI halides mounted in the fixtures. Pretty poor design. Retrofits are readily available which would keep your ballasts away from the halide heat of the fixture by keeping all ballasts remote. There is a recent thread posted where a reefers ballasts in her Outer Orbit fixture went out after only 13 months. The Aqualight dual 150 watt HQI unit with power compacts puts out higher PAR than the outer Orbit dual 150 watt HQI with T-5's as the Aqualight Pro HQI halides are driven by magnetic ballasts versus Outer Orbits HQI halides driven by electronic ballasts. However the dual 250 watt HQI Outer Orbit fixture puts out a higher PAR than the Aqualight Pro because the T-5's put out more PAR than Power Compacts as in the Aqualight Pro and they both have magnetic ballastsm making the halide output equal. However, when that higher Outer Orbit PAR comes because of the actinics it doesn't really benefit the corals it just gives a bluer appearance for viewing. Therefore unless you are using something other than actinics in the Power Compacts and the T-5's that will actually provide an edge to the corals such as 6500K to 10000K bulbs, or such, the issue of T-5's or power compacts does not really matter, unless you consider bulb replacement intervals and bulb replacement costs.
The Aqualight Pro is abetter quality lighting system mechanically than the Outer Orbit, but it is also several hundred dollars more expensive. I have never heard of any substantial problems with Aqualight Pro lighting Systems. I own many of them, but I honestly wish they were made with T-5 actinics instead of power compacts, but I would want the ballasts to be remote, where as I have never had a Power Compact electronic ballast burn up even though they are hood enclosed.
Basically, in a combo fixture it will be the halides that provide the light that matters to the corals, and the rest is merely viewing preference and either the PC or T-5 can provide that.
With an all halide fixture versus an all T-5 fixture, or an all power compact versus an all T-5 fixture things change. Then there is a totally different set of parameters and arguments. Basically the safe argument is T-5's with individual reflectors trump Power Compact lights which can not be efficiently reflected due to their shapes. Do to this fact lighting bulb manafacturers have not bothered producing the Power Compact bulbs in the full range of K values as they have with the T-5's.
