You should be a Magician if you plan for your main contribution to most situations to be spells and spirits.
You should be a Mystic Adept if you plan to be an Adept who augments their Adept abilities with magic.
What you want to not do is make a Mystic Adept who is half adept half Magician and tries to do nonsynergistic things like have a bunch of weapon skills, no buff spells, and a bunch of combat spells.
Check the link in my sig and look at the Transhuman Mystic for an example of a mystic adept.
In general, you want to make a Mystic Adept by figuring out a character that works well as an Adept, then figuring out how to back it up with magic. For example, make a B&E adept, then back it up with useful B&E spells. Or make a gun-combat-pew-pew adept, then back it up with buff spells. Or make a social adept, then back it up with Increase Charisma and mental manipulations.