"Ho" and "Po" are both words to show respect and are used about equally.
There is no rule about which one to use and when. "Po" may be considered more formal. Maybe that's because we might initially tend to use "po" when the seniority of the other person is obvious, but once that acknowledgment has been established then using either one would not matter anymore.
In situations, like addressing a jeepney driver or any other stranger inside the jeep, "ho" would serve as a generic respectful expression in the sense that age difference need not considered. A "po" might be unflattering to those who may not be that old.
In general though, it does not really matter whether we say "po" or "ho".
When it comes to using "po!" and "ho!" in responding to a call, for example, a parent calling a child who's at a distance or in another room, we might resort to using "po!". This is because it might be easier to say it aloud and the "exploding" 'p' might help project the sound further than aspirating it with an "h". (Note: these "po!" and "ho!" responses are just the clipped "opo!" and "oho!", which are the respectful "yes!" responses)