Well, it's actually the other way around for the most part, but they're 99% the same anyway.
Pit's arrows beat Dark Pit's, no contest. Although DP's guiding bow custom helps quite a bit for some matchups. Pit gets arrows he can camp with and hit the opponent basically anywhere, DP has to choose one or the other.
Their Sspecials, or Arms, are more or less equal. Upperdash Arm sends people nearly straight up and always KOs at 140, while Electroshock sends people at an angle and can potentially KO at 120 on the stage. Since they have SA on the moves, they can beat out nearly any recovery if timed right. Pit's will just launch people up, but DP's will send people out and potentially gimp or KO those recovering. Pit's KOs 10% quicker for every platform up on BF, but DP's deals electric damage as to hold an opponent for a teammate to strike him. Yadda yadda yadda... And the custom Sspecials basically only effect the distance traveled and considered to be not worth using.
DP has a stronger striking flight custom (I think...) , not that you'd ever use it. Their recoveries are identical otherwise.
As far as A moves go, the only one with any difference at all is Ftilt. DP's has little knockback, almost little enough to combo properly out of a sourspot. Pit's has high KB, enough to KO at higher %s. Considering they both have problems KOing, Pit is the clear victor here.
All that's left is their Final Smash, and that's up to personal taste.