Main things to consider on ballasts...
Start Method - You want a programmed start ballast for T5HO to maximize bulb life. Other types aren't going to blow bulbs up, but will affect the life & performance of the bulb.
Ballast Factor - This indicates if the bulbs are under or over-driven as compared to the T5HO spec. A Ballast Factor of 1.0 means the bulb(s) are run right at spec, so the bulbs advertised color ouput, PAR output, & bulb life time should meet or exceed them. Generally, 0.97-1.03 is acceptable before making a notable difference. Under 1.0 would be under-driven, over 1.0 would be over-driven.
Power Factor - Indicated ballast efficiency. The closer to 1.0, the more efficient the ballast is.
Bulb Quantity - May seem simple, but can easily be over looked. Make sure you're comparing apples to apples, as one ballast may fire 4 bulbs, another only 2 bulbs, etc.
Reliability - Basically comes down to the brands quality. I'd agree with gus6464 that the Philips ballasts are the cream of the crop, and HEP Group being a solid runner-up. I say this as someone who runs the HEP Group ballasts and are extremely happy with them. But going by specs, the Philips have a slight edge. Going by name, let's face it, Philips have been a major player in lighting for a loooong time. Can't go wrong with either of them though, and honestly last I looked it wasn't worth the time looking anywhere else but these two IMO.
