I think for footballing reasons alone, Fabregas would be better off joining United.
The only reason I'd see him go to Arsenal is because of the best.
No offence to Arsenal, but under the right manager, United are a much better team in my opinion, and I do believe we will invest heavily in the summer.
Depending how things go, yes we are out of the CL right now, but Arsenal could easily miss out next season, and to me, could be out for a few years to come.
As for improving. The central midfield of Arsenal is stronger, but they are weaker in many other places. Slightly improving in midfield, or improving more in other areas is the choice they'd have, but surely other areas are more important.