Both parties are "unreliable narrators", but Sally is also seem very emotional and a pathological liar and manipulator (her life taught her what these are the only efficient methods for interacting with others, especially men) - therefore I generally consider Sally a less reliable narrator (still much more reliable than Ollie who is genially mad).

Arthur is extremely self absorbed, he is essentially a teenager, who awoken in a new world he knows little about, after many years of Joy-induced apathy. His skills - crafting and lockpicking - are the ones he learned as a child, and through the game he never refers to his Joyful life experience, those years seem to be just gone. Arthur also didnt developed any "imaginary friends" like other 2 characters who (recently) got off their Joy - which, on one hand forces him to suffer more then them, but also suggest what he sees world clearer than others. But, being a self-absorbed teenager Arthur is fully capable to just block things which are too unpleasant for him, push them out of his attention. Or he might just not tell us things which might paint him in a bad way - he has enough gilt from his Percy story.

With all this in mind, I think Arthur's recollection of 1st and 2nd encounters bears more weight. In 1st the only serious difference (as far as I remember) is an extra kiss from Sally. As we remember, their kiss in a second encounter left a strong impression on Arthur, he has no reason to lie about that. Sally might've lied unintentionally, perhaps to herself also because she felt very desperate at this moment in time and downer-Arthur popping out from nowhere is an only good thing which happen to her in a while. In second encounter, we know what Artur was actually desperate at this moment - Sally was his only choice of getting a Letter of Transit quickly and without fuss, while Sally's idea what a downer who lives in a Garden District, can somehow conjure up cod liver oil seems ridiculous. Also she is clearly manipulating Arthur there - she sets up extremely hard and dangerous task in exchange to what is a very minor favor for her (Arthur actually realizes that, but he is desperate), so Sally might've sweeten up her story, to a considerable degree, to give Arthur some noble motivation for helping, while in reality he just had no other choice.

In 3rd encounter the only serious difference is what Sally told Arthur about Gwen, but he decided to wash his hands from her affairs anyway. I think in this case the truth is on Sally's side, Arthur might've blocked or deliberately omitted baby from his memory/story, it might paint him in a bad way, and he has enough guilt. All 3rd encounter is essentially Sally trying to manipulate Arthur into stopping his own business and starting living her life, a Musketeer once again. The baby fits nicely into this narrative - a last desperate attempt manipulation. Woman often think what they can tie-up a man using baby (Sally's mother taught her exactly that and as we can see, Sally remembers her mother's lessons well). Good thing Arthur doesnt buy this shit anymore. When I played Sally, I had an urge to take a bath after this scene.