If I know "Tap Personal Mana" and "Tap Ambient Mana", when I do decide which one is "active"?
And for how long?
For me this is chosen at character creation. Meaning, the way that character does this normally is determined by how they were trained to. But, the other option could be available after some training so they have another way if they need to for whatever reason.
For example: The wizards of the Korveck Academy all learn to "channel" the ambient mana of the world around them in order to cast spells. As one gets more experienced, they are able to handle more mana at one time (higher PP spells and/or more scaling) and per day (increased PP total). Only when they have attained a certain level of ability (lets day 10th level), are they allowed to learn the secret of storing mana within themselves for when they may be cut off from ambient mana. (This is easily reversible.)
While it may not be exactly "by the book," I just like this method for the flavor aspect.