Harmonius, you make no sense. With all the things your sovereign had going for her, she could have made the kill, almost as easily and with more reward, with a practice sword, and it would have not been much harder with a rusty sword.
And of course, unless you can stack damage (Burning blade, lethal line, swordsman line) you are better off with a more damaging blade. Hell, I cannot imagine a situation in which I would not rather have a skath blade, let alone an assassin blade, serrated anything, etc...
The Shadow dagger is one of the two weapons I enjoy finding early on. In my last play-through, I found both a Shadow dagger and a practice sword before turn 40. I ended up using the practice sword. Why? Because what I can kill with the dagger, I can kill with the sword, but I get 5-6 more xpt per garrote, so I do not see the point in killing them a tiny bit faster. And I am not sure about the faster.
Edit: the bonus xpt does not matter much for the sovereign, but it adds up for the spearmen and archers he is leading.