I disagree.
All good! Let's discuss further:
At higher levels, if the mystic adept keeps spending the karma for power points, the magician pulls ahead.
To be clear, pulls ahead at what? Both have the exact same offense potential, as they both have access to the exact same things. Strictly on the defense side, the mystic adept is completely unrivaled.
In order to keep the analysis focused, just consider Combat Sense. It is both a spell and a power.
At chargen, the mystic adept spends 15 karma to get 3 power points, all of which go into Combat Sense, giving him +6 defense and the ability to always make a surprise check. The mage uses one of the spells he learned (worth 7 karma) to pick up Combat Sense the spell. He also spends 12 karma to bond to a sustaining focus 6 and has to spend 24k nuyen (worth 12 karma), then casts until he gets 6 hits and now gets +6 defense and +6 to surprise tests. It seems like a good deal (15 karma to 31 karma)
Yes, the mage gets projection which is slick, but even in your own example here the mage is spending more resources to get his 6 combat sense dice than the mystic adept did, and that is before the mystic adept does the same thing (or quickens, the real winning move) to have double what the mage does since adept combat sense stacks with spell combat sense.
and the sustaining focus can be used for whatever spell the mage wants to sustain, so it's more versatile
Not exactly. Sustaining foci have to be dedicated to one spell category. I consider the use of foci less than optimal since quickening exits anyhow, personally.
If the mystic adept also took the spell to cast on allies, he would be paying twice, so not a good use of resources
Sure, unless you are planning to optimize defense. Shadowrun 5th is very clear about what does not stack, and that is Initiative boosts and attribute augmentation past +4.
If the mysad went with spell resistance at max instead of astral perception, mage could match that with a counterspelling focus, which is more versatile
Again not exactly, since counterspelling foci also have to be dedicated to one category. Plus the defensive minded mystic adept will have harmonious defense, which makes the issue of resisting magic leagues apart.
The mage will have standard resistance test for the spell in question, plus ranks in counterspelling, plus shielding, possibly plus a foci.
The mystic adept will have standard resistance test for the spell in question, plus max ranks of spell resistance, plus counterspelling ranks, plus shielding, plus willpower+magic+initiate grade from harmonious defense.
A couple hundred karma later, the magician and mysad now have MAG 8. Now the karma differential spell-to-power is 20 to 39, but I think the advantage still goes to the mage.
If a person makes a mystic adept with optimized defense for the build then yes, a mage of equal karma will be a slightly more effective spellcaster. The mystic adept will be untouchable outside of a horrible dice roll, or an adept with a high force weapon focus buffed by diagnostics from a technomancer sprite.