This isn't one of them.
First off, plate is misrepresented in your post and in popular media.
Backstory does matter, though. If a 20 is a veteran of the third war who took three tours through the frontline and your 90 is some up and coming recruit whose never had the misfortune to fight for his life, then I would expect you to defer to rank and proven ability. In fact, I'd consider it bad roleplay not to. Almost godmoding, in fact.
On the other hand, if you're both longtime vets, then I'd fall back to level to determine skill. Common sense should win the day. I'm still in favor of duels as combat solutions, but I typically only take that course when A. the other guy wants to duel really badly or B. my character is in such a situation that it would be a conflict of history to NOT fight. Fighting to the death or even to serious injury is not something to be taken lightly. It's not fun, and people don't do it FOR fun. Lurching to your feet and pulling steel for every "Your mom!" crack is not how life works.