I've had the privilege of studying western martial arts for a while. I'm not saying I'm some kind of badass swordsman but at least I have proficiency with the weapon
Anyway, if you look at a proper bastard sword, when you switch to two hands you can do more damage yes, but the biggest advantage is increased speed and leverage. This is even more the case with a longsword (note, a longsword is *bigger* than a bastard sword. The classical (older editions) one-handed d&d longsword is an "arming sword") where the longer handle gives you even more leverage. This speed and leverage is what leads to higher damage, but also higher increased *defensive* capacity, allowing you to deflect incoming attacks fairly well.
So that's "reality". I'm not really sure how to translate this into gaming terms though. Obviously you already get improved damage with the bigger dice. A +1 to hit, + 1 to AC springs to mind, but then that would be strictly better than defensive (although defensive would allow you to use a shield too).
This is all for 2 handed use. The benefit of being able to use one hand is whatever it is you are doing with your off-hand (shield, dagger etc), so it gives you options. It doesn't work better as a single hand weapon than a proper arming sword, so in my mind the effect would only work when wielding it two handed.
So let's compare balance:
Defensive: +1 AC but +3 with shield
Duelist: + 2 damage, +2 damage + 2 AC with shield
Versatile (2 handed): +1 AC, +1 to hit, slightly bigger damage dice (equivalent to +1 damage) *and* the option of using a shield (ie gaining 1 AC but losing +1 to hit and damage).
Seems balanced to me? Am I right in saying that duelist can use a shield (since it's not a weapon)?