T5HO lighting is about the best bang for your buck in my opinion. The reflector can be designed optimally for the tube to reflect almost all of its' light into the tank. A cross section of that ideally designed reflector is about halfway between a parabola and the McDonald's golden arches. When there is just one tube as in T5HO, the back side of the reflector can be optimized in design to reflect the light that shines off the top of the tube to reflect around and down and still go into the water. With a PC bulb, you cannot do that since there are two parallel tubes next to each other, so some of the light that shines up from each tube just reflects right back into the tube and is lost. That is called "re-strike" and it also why PC bulbs run hotter then T5's do.
A PC bulb is actually just a T5HO that is folded against itself in half. As a result, with PC bulbs, there is also light lost from the two tubes being right next to each other and each one shining light right into the other. That light is "re-strike" loss as well.
The cooler the bulb runs and the less "re-strike" there is the longer a bulb will last. T5HO lights usually have a 20,000 hr rated life, and they usually only drop about 10-20% in intensity during that time. PC bulbs last between 8000-15000 hrs (depending on brand) and the intensity drops much faster with time. You can actually be down to as much as 50% of the initial intensity within 6 months of use with a PC bulb.
T5HO bulbs are now cheaper then PC bulbs for the most part as well, especially if you are able to use the 4' length bulbs. You can find full spectrum 6500k 4' 54w bulbs now for around $10 each without too much trouble. The lowest I have seen on a good PC bulb of comparable specs is still around $12 each, and they only last half as long too.
T5HO will most likely continue to get cheaper over time, because they are being used more and more in hydroponic growing setups, and they are also being used heavily to replace older style T12 and T8 fixtures in commercial office buildings.
I wouldn't bother with T6 or T2 at this time. They are still pretty much a niche market that could be easily overwhelmed by the T5 and they could disappear at any time or just become even more expensive. They don't really benefit you anything over a good T5HO setup either.
Do a Google search for an "icecap SLR reflector" or a "TEK reflector" for a T5HO bulb. They are both very good reflectors. The icecap does a little better job of shining the light straight down below the bulb which provides a little more intensity at the substrate since the light is more concentrated. The TEK reflector disperses the same light out at a slight angle in each direction for more even coverage over the tank bottom if the lights have a greater spacing between them.
What Vaughn said about the T5HO being brighter is true as well and it is more then just lamp shape. T5HO tubes can actually be designed a little more efficiently (and the ballasts run a little more efficiently as well) so they can put out more light per watt then a PC bulb. The average 55w PC is about 3800-4500 lumens output initially, and the average T5HO is about 5000 lumens even though it uses one less watt. Now lumens isn't a good measure of usable photosynthetic light for planted tanks, but it does give you an idea of the intensity difference between the two bulbs.
Have a good one, Jeremy
