So I had this same question, so I ran some tests, and these is what I've come up with.
DP's arrows have a shorter range, and their direction cannot be changed nearly as much as Pit's, but they have a higher max damage. Both arrows charge and travel at the same speed, have the same min damage, and have the same cooldown.
DP's flight goes higher than Pit's, by a good chunk. For both of them the direction can be controlled in the same way, with the same number of different directions possible.
While Pit's Upperdash Arm sends the opponent up, DP's Electroshock arm sends them about 45 degrees up and outwards. Their range and speed appear to be the same.
Pit travels faster than Dark Pit, in both walking and running time.
From what I've found, everything else is the same. The only other difference may be weight, but it's harder to test that when it's just me here. The damage, knockback, sweetspots, direction, timing, and number of hits for all other attacks are the same between the two characters.
The damage for them both is seriously the same, I think people want them to be different so much that they think they've found something when they haven't.