The brand name is obvisously bigger (though I'm not so sure about the situation in Brazil - you probably know more than me here, but I was under the impression that Serie A was relatively popular in South America). However, you also have to consider that United are stakced with AMs: Di Maria, Mata, Herrera, Rooney at times, Januzaj, even Young. We've barely got one, so he's pretty much guaranteed to play substantial amounts of minutes. Add to that the already mentioned cultural issue, where I think it's safe to say that Italy would be more appealing than England, I'd favor us in such a scenario. Though the opposite wouldn't be far-fetched either, you're right.
Not that talking about this really makes sense, if he goes to United it will at least be partly due to them offering way more money to both club and player. The financials are an aspect where we definitely cannot compete with them.