When it comes to sheer composition, Magna Insomnia is on a whole new level from One-Winged Angel.
They're trying to achieve different things - and I'm not sure how you're defining "sheer composition", but Magna Insomnia can't really be said to be particularly complex technically when the series has had pieces like Nascent Requiem. The considerable difference in style between the series' final boss themes makes it difficult for me to really rank them in any sort of clear-cut way.
I found it a little overblown in context, this highly dramatic music underscoring a fight which is much less fantastical (by intention) than a lot of final bosses in the franchise. On its own, I like it a lot more, some very nice variations on Somnus (although I don't think it beats the same composer's "Darkness of the Unknown" from KH2).
Yeah, it wasn't the best fit for the scene it played over. I wonder if it was actually written for XV recently, or carried over from earlier on - since Somnus was with the project since it was first announced, I could believe that this arrangement of it existed in some form for quite some time, and was used in an unfitting context in the final game because it would have been the only chance to use it at all.
I hope they talk it out over cup noodles afterwards.
Invite Aranea as well.