What's the debate here?
In Elantris, the Aons can be used to work any arbitrarily complex spell. Teleportation, healing, changing appearance, creating food from dirt, it's all possible. For combat purposes, among other things, one can use an Aon to specifically target someone. Hard to dodge a homing missile or magic effects that can't even be dodged. (for instance, if you drew someone's name and then an Aon to cause death, would there be a projectile that could be dodged or would the effect simply happen? Probably the latter, and the only defense would be either protective Aons of your own, or maybe in Elantris there aren't any death Aons written into the city)
In Mistborn, all of the powers are very limited, and generally do exactly one thing only. You can't "write down" a hugely complex spell and activate it later.
I liked the Mistborn magic system to, but there's just no contest here. If an Elantrian is sufficiently prepared before battle and close enough to Elantris to be fully juiced up, there's simply no way he or she could lose. It's most likely possible to create a hugely complex spell by carving it into something and to set the spell to activate when you draw a unique character. So, by drawing one character, a spell could be set to teleport you around randomly, put a shield around you, block all mental effects, surround you with fire and whirling razor blades, and to automatically target any living being within 100 meters with everything you've got. Only real limit is that Elantris the city has a finite amount of total magic available to everyone, and individual practitioners can only draw on so much power at once.
There's a ton of weaknesses built into mistborn's magic. Among other things, metal users can only push or pull things made out of metal...in elantris, one could write a sequence of Aons to fire pieces of something not made of metal (like rock) at supersonic speed towards any of your enemies. Goodbye Mistborn.
As for making your own Aeon that activates a sequence of other Aons...I am extrapolating, but not by much. Elantris itself obvious has written into the city "translation" that somehow specify what each Aon will do in terms of a simpler form of a magic. Like a high level versus a lower level programming language. That's why the city broke in the book....a key library had it's name changed...
One final thing : Atium. Atium only works if there is a WAY to win. If there's no possible way to get close enough to that Elantrian surrounded by a shield and maybe whirling non metallic blades and protected by healing Aons set to repair any injury in seconds, you can't win.
And why fight at all? A clever Elantrian could simply have an Aon that teleported him home while replacing his body with an illusion that is booby trapped. That's certainly what I would do : the trap would blast anyone who tried to harm my illusionary body. If that didn't work, I'd return to the battlefield, invisible (yet another effect that is not available to a mistborn), and spend some time spying on my opponent to determine weaknesses. Or perhaps simply remote scry my opponent : presumably that's yet another thing that can be done with Aons.