In Arelon, there is no other rivaling magic, so an Elantrian's Aons are very powerful, despite the time it takes to draw them. The magic is about how good you are at drawing and how much knowledge you have about Aons.
On Scadrial, Allomancy and Feruchemy are far more about speed. A Mistborn is powerful if he/she learns to use his/her limited powers (limited number of metals) quickly and wisely.
Initially, my mind thinks that a Mistborn would definitely win, due to his/her speed advantage. When I thought about it, however, I realized it's not that simple. A truly skilled and knowledgeable Elantrian, given five or ten seconds to construct an Aon, could do basically anything, assuming he/she was able to stay out of the way for those five/ten seconds. For example, he/she could probably direct a fireball at a person with a modifier so that it would follow the person like a guided missle. He/she could teleport, which would allow him/her to sneak up on the Mistborn somewhat easily. The list of Aons is not necessarily finite, and we definitely don't know all of the things one could do with them. There are probably some ancient, dark, forbidden, very complex, lost-in-time aons that do things like control minds and cause automatic death. It seems to me like, if the Elantrian knows the right Aon and is able to draw it in time, he/she basically automatically wins.
Just thinking about those 'old, dark, forbidden, very complex, lost-in-time aons' makes me want to see an Elantris sequel...
A couple more somewhat notable things:
Presumably, bronze could not be used to detect Elantrians, because the Dor is an entirely seperate magic system than Allomancy and Feruchemy.
An army of Elantrians would have a better chance, but not against an army of Mistborn (if the fight is fair, both sides' numbers will be equal).
Dakhor monks don't have much of an advantage, seeing a they're basically just thugs, but they COULD distract the Mistborn while the Elantrians draw Aons.