I was wondering the very same. I'm under the impression Dark Pit hits harder and Pit doesn't have any noteworthy advantage, though I could be wrong.
Plus, his arm is way cooler. Out of so many arms in Uprising, why did they pick THAT for him....?
Dark Pit does not hit harder than Pit in any way, it's all in your head. In fact, his Ftilt (when tipped) does even
less damage than Pit's Ftilt at the tip, effectively making it KO later than Pit's Ftilt does.
The main two differences are literally the arrows and Side B, and with Smash Balls on it's his Final Smash that's significantly different.
His arrows are basically worse, only good for shooting one in the general space you
think your opponent is going to go. So, you might shoot it to force them to go low, for example. Or, you might shoot it to catch their jump. Other than that, you don't have enough angleability to... use it for meaningful edgeguards like Pit does thus making it much worse in the end (the trade-off of getting a max of 10% out of one isn't good enough either).
Side B is widely considered worse because it's taking away a good KO option off of roll reads and generally early KOs period. Pit's Side B hits people vertically and since most characters in this game are floaty, vertical KO moves are
extremely important to have. Dark Pit's is way more situational to use in comparison to that, since it sends at like a 45-50 degree angle and thus doesn't KO well on-stage at all. What its main use (that I've tried and seen) is for edgeguarding but it's such a RISKY idea that you often times shouldn't go for it unless it's certain MUs where a character's recovery sucks (Little Mac, Link, Ganondorf, Captain Falcon, etc). Mainly because if you whiff that Side B in the air, you're going to be putting yourself in a bad position and near the blastzones (you can come back but... if the other player reacts quick enough they can certainly keep you off-stage).
So, generally, Dark Pit is actually
worse than Pit based on these things alone. If you like him, you like him and are free to choose to continue to play him. But Pit has way better tools and KO options with his specials than Dark Pit does. Dark Pit is basically the Roy of this game, where he's actually
worse than the character his move set is primarily based off of. And it's particularly because Dark Pit doesn't have any meaningful changes in his normals that make him stand out from Pit to be... well... better than him in other areas -- he's strictly worse. I'd almost be tempted to say he's worse than Roy (in terms of clone characters and how different they are from the original and their advantages vs. disadvantages) because at least Roy had a clear advantage of KOing earlier than Marth with his normals and Flare Blade and how good Roy's Dtilt was for setting up juggles and stuff.
In summary: Dark Pit is no different from Pit other than his specials and Ftilt and because of those differences he's strictly worse than Pit. You, of course, can continue to play him if you so choose but know that you could be playing the better character of the two instead and have everything you've learned from Dark Pit transfer over to Pit instead + have the added benefits Pit possesses.