Take i.e. longsword 1d8/1d10 in two hands.
you get 4,5dmg one handed, 5,5 damage two handed,
If you have dueling style you do 6,5 damage one handed. one more than with two hands.
Even if you have both styles, two handed longsword will deal only 6,3 damage on average. Still less then dueling style.
I want to emphasise a point here. If you choose a 2H weapon, you are sacrificing the things a spare hand could do (like, use a shield) in return for using a weapon that does more damage.
Therefore, it is entirely appropriate that, even after Fighting Styles are considered, 2H weapons should still be doing more damage.
This does not need balancing in favour of 1H weapons!
This is illustrated by Versatile weapons themselves; a longsword (for example) does more damage when you use it in two hands than when you use it in one hand. This makes sense.
But with a PC who has both the 1H
and 2H Fighting Styles, suddenly he does
more damage using it in 1H than when he uses it in 2H. This does
not make sense!
And to add insult to injury, the 1H guy can flip off the 2H guy from behind his
shiny new shield!
In my games, Dueling Style does not work if you have a weapon
or shield in the other hand (just like in the playtest). If you want sword and board, choose the style aimed at the shield!
Also, I use GWF as written, rather than as intended, i.e. reroll
all dice of damage from that attack that come up 1 or 2, no matter the source of that damage (Sneak Attack, Divine Smite, Flame Tongue, whatever). It is still a worse style generally, but at least it has a niche where it can shine.