I think that Stoner has been one of the most consistent playoff performers out of the defensemen if not the most consistent. He has elevated his game a bit everytime, he knows what he can do and what he can't do. That said, Stoner hasn't played in like 6 months or so, which is worrisome. Could end up being a big mistake playing him over Theodore.
On the other hand, going against Preds, Theodore might be better option as the Ducks are going to need all the offence they can bring and Theodore has brought just that but he's also been responsible of a bunch of goals against so it's a wash. Those are invaluable lessons for him and being a smart player, he should be able to learn from them, the trouble is, playoffs isn't the time and place to learn these things.
I would keep both of them on a very short leash, maybe play Theodore in game 1 but if he doesn't perform well, give Stoner a shot. Putting him with Vatanen could end up in a solid pairing and allow Manson not having to babysit anyone and make the d core as a whole better.