Stats wise they are better then all 5* girls tho a few come close (looking at you Toad Lilly).
Skill wise they are better and often have high % chance at lvl 1 as well.

But where 6* girls really shine is their passive skills.
They are so much stronger at buffing your whole team that having just 1 can improve your actual combat power but as much as 30% or so.

Flowering Peach for instance gives +20% atk at 100% affection to all squad members (Not sure if all members have to have 100% affection or just her but as your main squad will all have 100% it doesn't matter) AND once evolved gives +15% defence to all squad members as well! No 5* even comes close to that kind of buff.

Ok, so Flowering Peach does have the best squad wide buff of the 6* right now but all 6* have either a squad wide 15% buff or a higher % buff limited to fewer girls.