I wonder if the premise should be questioned. I think if you take it as a given that you're definitely playing Preordain and Duress this turn, most of the time leading with Duress makes sense. This is probably true for ordering Duress and any card. The key factor is that opening with Duress reduces your opponent's ability to make the right choice. If they have a response like a Mental Misstep or a Brainstorm, Duress forces them to use it when you choose, where Preordain gives them the option of using it when they think is best.
This is probably more important than the information advantage. To evaluate the information advantage, you'd ask:
- what kind of information might Duress reveal that would change how I resolve Preordain?
- what kind of information might Preordain reveal that would change how I resolve Duress?
After asking those questions you should see the trap in the original question. The most likely way for Preordain to change how you would play Duress isn't that you might have them discard a different card, it's that's drawing a particular card might make you not want to play Duress at all. If this is turn one or two and you haven't seen your opponents hand yet, Duress is high value and it's PROBABLY being cast right away. But in many mid-game or later situations, you want to be careful you don't Duress away some medium-quality draw spell and pass the turn, just to have them draw a counterspell later. You want to cast your Duress as close as possible to the turn you try to win (which is probably not this turn if you're worried about a Preordain getting Misstep'd). So I think another question to ask is:
- If I led with Preordain, what could I find that would make me not want to play Duress this turn at all?
If there's a pretty reasonable chance you'll find one of those cards, leading with Preordain is probably best. The other scenario I haven't heard mentioned in the thread is if you want your Preordain countered. If your hand isn't particularly reliant on Preordain then you may not care if it gets Misstep'd. An obvious example is if you have an Ancestral Recall as well, but it could just be the relatively common case of "your hand is totally fine, you're not missing anything critical, Preordain would just make it better". If you lead with Preordain and they Misstep it, then follow with Duress, you may be able to answer two key cards instead of one, which could be significantly better than whatever Preordain would have gotten you.
In some hands that line is particularly strong. If this is turn one and I'm talking about this line with a hand of Preordain, Duress, Underground Sea, Mox Sapphire - I'll hem and haw over my hand for a while, sigh and reluctantly keep, drop the artifact mana without playing the land, and sheepishly run out the cantrip. A lot of players will be extremely tempted to "get you" from this game state. If you can sink your turn one Preordain into their Force of Will and follow THAT with a Duress? The game is usually over.
Ultimately I think my heuristic is:
- Do I want my opponent to interact with the Preordain? (probably fairly common, easy to notice)
- Will the results of this Preordain make me choose not to Duress? (rare, but easy to notice if you're looking)
- If no to both, just Duress first
Those aren't the only factors to consider, but for me those are things that I can reasonably figure out in the middle of a tournament game