I like Pit but if I put Marth next to him I wonder how Pit got this favor that Marth didn't. Is it the Nairo/Earth boost? Because frankly it feels like you can abuse the exact same **** on Pit that you will to Marth. Excusing pit having a projectile and a *laggy* reflector.
I don't think Marth is bad at all (I had him at #25 in my rankings), but if we're comparing the roles of Pit and Marth, Pit is way better equipped at dealing with characters with projectiles, and I'm not really talking about just "having the Guardian Orbitars".
Marth's best match-ups tend to come against CQC characters, where his always present optimal reach can space out close-range assaults and weave in and out of approaches. However, Marth has struggles against characters like Villager and ROB, who can sit back and pretty much force predictable running lanes for Marth. Marth has no real answer for this, other than to play
extremely patient and hope for a mistake/opening. Strong (re: good) projectiles are good against Marth.
Pit has tools to deal with these projectiles. Guardian Orbitars is one such tool, but as you pointed out, it's not fast and takes some timing practice. Upperdash Arm, however, is a second tool against projectiles, and is arguably way more valuable in certain match-ups. For example, Villager pretty much can't put up a Lloyd game whatsoever, as Upperdash Arm reflects Lloyd horizontally, goes through Lloyd, and gives Pit a free hit on Villager
on top of the reflected Lloyd if Villager is close enough. Multiple jumps give Pit a way to stall out projectile games as well, something Marth wishes he had.
Pit also has a much better gimping game, something very valuable in this game. He can gimp people at safe distances with his projectile as well, which such security is a luxury not given to pretty much any other character. Dark Pit doesn't have as much gimping potential, though, which is one of the major reasons to use a character like Pit over Marth, who has greater upside. Hence why, IMO, there's clearly some characters that belong between Pit and Dark Pit when creating rankings.
Pit's increased range has been advertised ad nauseum in this thread, and rightfully so. It's much improved and gave him a new way to tackle specific threats. However,
Pit's ability to use normals to space mid-game is not his defining feature. Pit has been, and will be, a tools-based character who has something in his quiver for pretty much every match-up in the game. Marth's match-ups are much more polarized, and although Marth has way more KO potential (making him a powerful pick), he's not in the same capacity as Pit. Pit can cater his playstyle to his opponent, and any opponent, to a major degree of change. Marth cannot.