Going to a ready made winner, shows a lack of ambition in my opinion.
Go to Barcelona, a club that is and will continue to win La Liga constantly. Or go to a sleeping giant, and help be a catalyst for their return to glory? Where will you be a bigger legend? Which is a bigger accomplishment? It's the difference between being the architect of your own success, and piling on someone else's. United is a perfect club for this right now. We've got the money, we've got the global profile, we don't have the results. If you're a member, no, a driving force for the team that brings another Premier League title, you instantly become one of the biggest names in the clubs entire history, and by extension, due to Uniteds global stature, you become a legend of the game.
Go to Barcelona? It's a sure thing. No risk. Yea you are going to win a grip of La Liga titles over the course of your career, and maybe some CLs as well, obviously that's tempting, but if you're truly ambitious, if you truly believe in your ability, you're licking your lips at the prospect of helping to return United to elite status.