Huh, didn't know this discussion thread was already a thing. Ironic, given that this was a matchup I came up with on my own a couple of years ago.
Initially, I would have thought that Satsuki stomps. Her ability to make Ep3 Ryuko into a casual large building level feat, and her scaling to Senketsu (hence, the opposite of Junketsu, allowing for parallel feats by the end of the series) would make this an easy win for her at face value. That said, I've recently been thinking about Morag and how she's portrayed in the scenes that we pull feats from, and I realize that it's not Morag that we should be thinking about
per se, but Brighid.
See, in the scene in Mor Ardain (the one where Poppi is mistaken for Lila), Mythra states that, remembering her sparring(s) in Torna, she is confident that Brighid can box her, or is at least relativistic in skill and ability.
(time as any to mention that I did take a cursory glance at Morag's wiki page, and yeah it does mention Brighid in TTGC scaling to Mythra)
Mòrag Ladair is a playable character from Xenoblade Chronicles 2. As the Special Inquisitor of the Ardainian Empire, she was initially introduced as an antagonist with the intent to capture Pyra so that the past wouldn't repeat itself. After two battles with the heroes due to misunderstandings...
vsbattles.fandom.com
There is also the Heart-to-Heart in the Land of Morytha, where Brighid mentions the records in her diary - and notices that, while her loss ratio would be quite damning, A) there was one win to her name, and B) that all (or at least a majority of) her losses were considered to be quite close.
"You gave it all you had, but I guess I just have certain attributes" - Mythra.
Now, having absorbed that exculpatory evidence, what are the most reasonable Mythra feats that we can apply to Brighid (and by extension Aegaeon + MAYBE
Morag herself)?
1) Light Speed.
This is a given. This is essentially Mythra's bread and butter given her element, and the fact that Jin was a related entity where acceleration to light speed is clearly defined. While you may see people disagree with the nature of Jin being light speed given how underutilised that ability is by him by the end of Base XC2 (and even I have to admit, the game is... less than great as a power scaling case study), the fact that this is even a prospect to the player is solid enough evidence that this is speeds that can at least be tagged (yes, tagged. Brighid has not shown these feats herself, and it would be unreasonable to assume otherwise).
2) Multi-continental power feats.
This one I'm a little torn on (pun not intended). While it is true that Mythra was forced to go apeshit in order to stop Malos and his Siren that he was using to casually put down continent-sized titans, the nature of Mor Ardain and it's political dividends (or lack thereof) would imply that the ability and aptitude of compatible drivers had shrunk dramatically compared to 500 years prior (as proven by the observations of the party in Gormott, and the fact that Mor Ardain's Government were willing to move their capital there).
I bring up Mor Ardain's current affairs because author intent is not unfamiliar to power scaling as a narrative device. It would either imply that Morag is just as competent, but a rare breed, OR it would imply that the best that could resonate with Brighid and Aegaeon were at a lower minimum standard (likely the former). This is also a reason that I'm hesitant to bring Aegaeon into this at all, not because he doesn't scale (he does), but because outside of TTGC justifying the imperial cores direct scaling, he is straight up deprived of data on the matter.
With all of that said, I do think that we have other advantages through the degrees of separation that scale to Malos, implying not just potential multi-continental feats, but even planetary. In fact, it is not unreasonable to believe that the Siren(s) that Mythra and Malos control scale to the Aegises - an expansion of their abilities to boot.
With all that mentioned...
I think this matchup is a toss-up. How you interpret Morag and her situation that gave her access to Brighid/Aegaeon's powers, vs Satsuki having her own continental feat(s) misinterpreted, but having more flexible and situational abilities to compensate.