Are we looking at this as a stand alone season or the future?
If Arsenal win the league next season and United win nothing and remain outside of the top 4, then Arsenal. Although that is a weird way of looking at it.
I guess what I was thinking is that Spurs finished second a few years ago, 2017 or 2018? and no one cares. What matters is what Arsenal go on to do, next season. If we win nothing, then United had a better season.
Looking at the season in isolation, I'd take the FA Cup tbh. It's another trophy and record and that is what football is all about. There's no guarantee that because we finished second this year, we will do well next year but a trophy is a trophy.