Barcelona and Manchester United have good performance in domestic league and consistency at the winning trend except Barcelona held draw by Espanyol at first match after FIFA World Cup 2022. Current performance from Manchester United have quite well and win all matches after World Cup.
I don't have ideas about which one possibility qualifier to next round in UEFA European League, however between Barcelona or Manchester United have eliminated at playoff round. Nowadays away goals nit important thing after UEFA removed all rule about advantage of away goals.
Both teams have been in good form so far in their respective leagues, but that's just something that might be different when they meet in the playoffs. I tend to think Barcelona have something to stand for in this playoff game, but as far as I've seen it I think Manchester United have also improved since the arrival of Ten Hag.
It is difficult to predict who will qualify for the round of 16 because both of them have the opportunity to win matches in 2 meetings. Each of them has to take advantage of the home advantage, so you don't really know who will win a ticket to the round of 16.