I think for speed a saber would be better. An arming sword is slightly better thrusting and the extra edge is nice, But a saber is better optimized for cutting and slashing, And it's shape is more aerodynamic resulting in faster movement and speed. Against armor both will have similar results either way. What do you guys think?
My personal taste comes down to usage. Am I just toting it about? In that case I'll gladly take a sabre. If I'm carrying with consideration of using it I'll go with a broad sword. (I'm assuming that's what you meant by knightly). Of course I would see myself moreso toting it about if anything, To show off, So the sabre is what I'd carry.
The big thing to remember is that a saber is used almost exclusively on horseback for the reason that it doesn't get caught in the bodies and is more aerodynamic, While it suffered from a lack of armor penetration. The sword (broadsword), On the other hand, Is a footsoldier's weapon and is meant for the purpose of stabbing not really slashing. The Arabs and Turks were famous for using the saber on foot because they fought an enemy that wasn't very well armored. It was effective for them to slash quick with a lightweight weapon against an enemy wearing mostly just plain cloths. This is contrasted with Europe, Where the broadsword was used to both stab and slash. The stab was to pierce thick armor while the slash was for the lightly armored. So in terms of general usability, The sword tends to beat out the saber.
A polish saber is indeed nice for cavalry fighting, However, The Arming sword is much better in infantry and sword fights. Thus, There is no real comparison between these two weapons. They are both designed to face different and specific scenarios. Which makes this question totally nonsense. Finally, There is need to tell you that while the Polish saber is a representation of status in Poland, The knightly sword was used much more in the courts of Germany, Britain and also France. (During Middle Age of course)