Have to say I'd prefer Van Gaal. Ancelotti is calm, not disruptive, doesn't necessarily feel the need to make big changes and shake things up when he arrives at a club. And all of that is great, but I think this club needs shaking up. I'm not talking about the players - aside from the midfield and the need to replace departures this summer (Giggs, Ferdinand, Vidic, maybe Evra), I'm happy with our squad. But the style, the approach and the mentality all need disrupting. We need someone like Van Gaal to come in, with a bit of arrogance, and rough everyone up a bit, really impose himself on the club at every level.
Plus Ancelotti's natural style tends to err towards the defensive. Obviously that's not true of his current tenure at Madrid - he's flexible enough to know that caution doesn't suit the embarrassment of attacking riches at his disposal. But even Mourinho, the most defensive of all the current top coaches, played attacking football at Real, so that hardly demonstrates anything. Van Gaal's philosophy strikes me as more suited to United.
Wouldn't complain about either though. They're both top managers, which is what we need.