arrows are bad regardless and force the same situation no matter which one you use, it's only for forcing airdodges because they are going to avoid it offstage. You'd think pit's arrows would be more effective against characters that recover low but it doesn't do anything really.
Side b being a kill move is a pretty moot point considering it's just as if not more unsafe than an fsmash which will kill earlier anyway, dark pit's not only does more damage but puts them offstage which is a position he can deal with very well and get early gimps rather than praying your opponent is dumb enough to not shield at 150%. Not to mention dark pit's side b is way way waaaay better against rosalina, maybe some light characters can get cheesed by pit's side b but that's about it.
ftilt is a bad move so who cares if it does a little more damage, you shouldn't be using it in the first place.
Pit is a better character on paper, but there's really nothing but preference in which one you use, it doesn't make much of a significant difference outside of a few MUs to make it to the point where you wish you were using one instead of the other, other than having to listen to pit's horrid voice, that alone should make that **** not worth it
tl;dr - If you can use one pit, you can use the other, it's like having a character with custom moves while no one else does basically, you can pick the one that feels the most natural in a MU