I prefer the theatrical ending because...

I've just watched this for the first time. I loved it and I wish I'd seen it sooner.

I actually watched 4 different endings: https://youtu.be/NeV0MvU1Uhs

My favourite is number 3: the Open Ending - stuff is resolved, Evan has to live with the choice he made, but it's not all necessarily bleak for him.

2: The Happy Ending was similar but nicer, but maybe too nice; too cheesy.

Of the theatrical and director's endings... whew! Tough call. I think they're both great.

The Director's Ending is a logical choice from the character from where he is at the time. It's shocking, if not surprising and it is much neater. But bear in mind he didn't choose the film he watched. He only had time to watch whatever was loaded into the projector already. With this ending, his options as a foetus were limited to miscarrying himself or... doing nothing. What else could he do? He didn't have many choices and that's less interesting to me.

I prefer the theatrical ending.
Again, he didn't choose the film, but once there he had myriad options: he could have tried to say something else to Kayleigh, or her dad, or brother, or his mum to try to make things right. But he chooses to alienate Kayleigh. Evan has options and he decides to make a painful choice, then has to live with it and it's entirely selfless: no one but him really gets hurt, not even his mum, as she does in the director's cut.
And then, he has to make another similar painful choice years later when he passes Kayleigh in the street, and live with having chosen to stay out of her life again.
It's bleak as all hell for him, but I find it more interesting than the director's cut where he makes a difficult choice once, -the only one available to him if he doesn't want everything to play out the same with Kayleigh dead and him helpless in the psychiatric hospital, and where he doesn't have to live with the consequences himself.

It's late and I may be rambling so basically, what I'm saying is:
Both endings are essentially: Remove himself from the equation so he doesn't inadvertently ruin someone/everyone else's lives... but in the theatrical version, he has to live with that and I think that's a more interesting note to close the story on.