Realistically, the only time the Mystic Adept is better than the Magician is when one is making a certain style of combative mage character (even by the book). They do have combat spells, but the Mystic Adept combat mage is better served having his combat spells as backup "tricks up the sleeve" and primarily using his casting for self buffs in conjunction with the Adept powers.
For more utilitarian, less combative mage characters, going full Magician is better because no matter how much the nay-sayers want to poo-poo it, Astral Projection can be a pretty potent ability. In these cases, thinking with the BS "mage+" idea that people threw around in the first days after the PDF release and going Mystic just because of that not only sacrifices this rather potent ability, which isn't always applicable granted, (neither is being a dead-eye shot with a Panther Cannon or being a national politician level Face by the way) but it will be far, FAR easier to spread oneself way too thin and end up being nigh-useless.
As to the Essence Loss question here, there's not really any clarification needed. The rules say you lose a Power Point when you lose Magic period, so you would lose one even if you had fewer Power Points than Magic rating.
