Ideally the light bulb you use is as long as the tank, so you get the most uniform light in the tank. T5 bulbs come in sizes that match many of the standard tank sizes. PC bulbs rarely ever match the tank length. But, for odd size tanks, PC bulbs can be just the right length. And, for a ten gallon, 20 inch long tank, no T5 bulb that's commonly available matches the tank length, so PC bulbs can be a better fit to the tank.
T5HO bulbs are very bright bulbs, really too bright for tanks under about 24 inches in height. PC bulbs aren't quite as bright, so should be a better "fit" for tanks under 24 inches in height. If cheaper than AH Supply PC fixtures are used, they are even less bright, and can be very good for low height tanks.
T5HO's rely heavily on their reflectors to get superior penetration. As an example Hagen's T5's aren't going to be as bright with the same bulb as would a fixure that utilizes TEK or IceCap SLR reflectors.