I choose based on where I am coming from/going to in London. The easiest way to Hyde Park is LHR and then the Piccadilly Line on the Underground, and that means you can use the Arrivals lounge as your hotel may not have a room free until the afternoon.

LGW has the better aircraft although the seat is slightly smaller. If you do want to do that it's not too difficult to get the Gatwick Express to Victoria and walk/taxi from there.