Why can't you just go offstage with Pit then?
Pit's a strictly better character, there's nothing Dark Pit does that Pit doesn't do better. If you're losing more games with Pit than you are with Dark Pit... Then I am judging.
Actually, Dark Pit KO's about 15% earlier than Pit does with his Side B if you're near a ledge. Also, his attacks generally do more damage, if only by a little.
Sure, Pit can KO more reliably with his Ftilt and Upperdash, however, those two things alone do not make him "better" than Dark Pit, overall.
Just as Sakurai said, it comes down to personal preference.
Oh, and btw, I've smacked around quite a good number of Pit players with my Dark Pit, some who I would even call masters at him. While I won't go out of my way to say that I'm one of the best Dark Pit's in the world, I will say that he is an equal when compared to Pit.
PS: I don't think I've challenged you to a match yet, I've been on this little fighting spree with all the managers on the site. I've taken down a fair few of them. So, if you'd like, we can play a few rounds. Maybe I can show you just how potent Dark Pit can be.