offers more connections (more flights from the US and more destinations in Europe/Mid East), however i would say LGW
is easier to find your way around. In LHR
you could arrive in Terminal 4 and depart from Terminal 3, which means a bus journey all the way around the airport and then through the tunnel into the main terminal area, which can take up to 15mins. You could however get the tube, but then you have to wait for it to arrive etc, and even that could take you 10mins. However at LGW
if you're arriving in the south terminal and departing in the north terminal then you just need to hop on a 40 second skytrain ride and your there. And 2 trains operate so you would be waiting no more than 1 or 2 minutes.
Also if you're going to be connecting in the morning then the security checkpoint, expecially in LHR
's T1 is going to be mayhem. When i flew to Madrid, an 08:15 departure, it took us nearly 45mins to get through security. Gatwick, i have never waited more than 5mins.