There are two midfielders being linked with them right now. One is William Carvalho (ca. £35m) and the other is Vidal (ca. £45m-£55m).
Vidal is a box-to-box midfielder who would be wasted stuck behind the halfway line, but Carvalho is a holding midfielder, so on paper Carvalho makes more sense because Herrera isn't a holding midfielder. Herrera has been more of an attacking midfielder for Bilbao but my guess is he'll play deeper because of United's attacking options and will be next to a holding midfielder.
Or maybe that's me being wishful, because Vidal is good enough to transform United in many ways.
Carvalho/Herrera seems like a decent, well balanced and youthful midfield combo, but certainly not better than City or Chelsea's options. Carvalho looked quite slow and ponderous in the World Cup but people who watch Portuguese football say he's a top talent, so my doubts are probably wrong.
Worst nightmare = they sign BOTH of them and field a Carvalho/Vidal/Herrera trio.