@alster52 - they are not better or worse. They are different.
The young nobles can train into two paths--rogue kingdom knights or hero adventurers. Comparing the two, Sarleon for example, a Rogue Sarleon Knight is cheaper and faster to train than a Sarleon Knight, but is also weaker. A hero adventurer is more powerful than a Sarleon Knight (and is a horse archer), but is also much more expensive to maintain. So it depends on what you want--if you want a decent melee knight quickly, go for rogue knights. If you are willing to spend more time and money training, go for a kingdom knight or a Hero Adventurer. And sometimes, you may not have a choice--you just have to use the units you find in a tavern or in a prisoner train.
And not all of the rogue knights and kingdom knights are the same. An Empire Knight is much easier to find and train than a Sarleon Knight, but is weaker.
If you want a general ranking, ignoring price and training difficulty, it's this:
Hero Adventurer > Sarleon Knight > everything else is pretty similar. HAs and Sarleon Knights are both top-tier units, I'd call all of the remaining kingdom knights (except Pendor Black Archers and Pendor Foot Knights, which are specialized troops) and all of the rogue kingdom knights roughly similar. Rogue Blackheart Knights are also not as powerful as the used to be after the nerf to the Runed Bastard Sword.