Well, you get a better PAR and lumen value with T5HO lights then you do power compacts. T5 lights also run cooler and use less wattage then Power Compacts, so you get the biggest bang for your buck.
T5HO lights are smaller, approximately 1/2" in diameter, and T6HO lights are approx. 3/4" indiameter. Essentially, you can stuff more T5's in the same space then you can T6 lights. T6 lights are also not very well known just yet, and therefore it will be a very special order for your LFS to get them in if you ever have an emergency.
If I were you, I would go with the T5 lights with individual reflectors. Try for one of the Nova Extreme fixtures.