There's clearly a few different ways to interpret what's being asked here by the looks of it.
Some are looking at how enjoyable the season would have been for the bulk of it, with Arsenal having a much better league campaign and so enjoying the bulk of it much more.
Or looking at how far the team is down the 'project route' / how close the team are for challenging for the two biggest titles (PL and CL). And, yeah, on both of those counts Arsenal come out better than United (though haven't actually won the former for 20 years and never won the latter!)
However, by the end of it all, Arsenal once more ended a season trophyless. While United managed to 'rescue' their season by winning a major trophy and qualifying for Europe next season after all.
And so that makes it all much closer, and becomes the regular 'CL qualification but no trophy v No CL qualification but actually winning a major trophy' debate.
Personally, I'd always choose winning a proper trophy above the 'top 4 trophy' that Wenger used to be mocked for but has now become a much more commonplace view. So, at the start of the season, if offered the two options, I'd personally have chosen finishing outside the top 4 but winning a major cup.
Because for me a season with a major trophy success is always preferable by the end of it to a trophyless season but with CL qualification. Though, admittedly, until that trophy success is achieved, the season might not feel as good along the way as a strong (but ultimately fruitless) season like Arsenal's.