It's Orr with some distance between them. It would be a great argument if Lemieux didn't miss over a quarter of the season, but he did. With Orr you get an elite defensive player for a full season, with Lemieux you get little to no defensive value for around three quarters of the season. Orr is likely the best possession player ever, Lemieux in 1993 was no slouch but he isn't on Orr's level in that regard. Lemieux's only edge is on offence, and he scored 21 points more than Orr did. Lemieux outscored Orr by 15%, which is obviously impressive, but he also played in a league where scoring was 16% higher. Orr outscored the number ten scorer that year by ~83%, Lemieux outscored the tenth leading scorer by ~30%. Orr's gap would grow if you removed teammates for both of them. Even accepting that Lemieux played in a strong league I'd say that offence is pretty close.
Orr played with the best offensive player that either of them played with that season in Esposito, but I think that Pittsburgh was a more talented team. Lemieux's linemates were all star level players when near their peaks (Stevens and Tocchet) and he also had a HHOF offensive defenceman in Murphy, plus had Francis and Jagr available for power play duty. Boston also had a deep and talented group of forwards, but I don't see any HHOF players other than Esposito without Orr's influence. Lemieux did play in a stronger league than Orr did, so I suppose he can have something of a boost from that, but there isn't much else Orr could have done. Boston had 7 of the league's top ten scorers that year, basically Orr and two forward lines, and the main thing they had in common other than being Bruins is that they all got to play with Orr. Both absolutely dominated at even strength - Boston dominated to around 3.2 goals for vs one against whenever Orr was on the ice at even strength, Pittsburgh scored around 2.3 goals for to one against when Lemieux was on the ice at even strength. Lemieux is probably the greatest power play player ever while Orr was merely great, so that's an edge for Lemieux. Orr was a much better penalty killer, which is a bigger edge.
To put it more succinctly, I'd say that Lemieux has a small edge offensively but Orr comfortably beats him outside of that. It's a very interesting comparison if Lemieux had been able to play the whole season. No shame in an all time great season and probably the most impressive, all things considered, season I can think of in any sport that I follow.