Just want to point out to those unaware of Gatwicks transport choices, you can take trains to various parts of the south east that you can't directly from Heathrow, and you have more choices in getting to central London and cheaper than the Gatwick Express.

I live on the east side of town and take the Thameslink service to London Bridge station before connecting to the underground. There is a train every 15 minutes on average and it takes about 30 minutes travel time with only 1 stop. I've never had any problems with getting a seat or finding space for bags even at rush hour, but then maybe I've been lucky!