Gatwick generally sees less delays than Heathrow and also tends to have shorter immigration queues and slightly less crowds but it is further away meaning journey times into central London are longer and cost more money.
Heathrow has the convenience of being located within London and connected to the London Underground so a London Transport Travel Card (or Oyster Card) covers the entire London trip, providing you take note of travel zones.
There are just 2 terminals at Gatwick as opposed to Heathrow's 5 terminals which can make arrival/departure easier for nervous passengers who may worry about finding their way to the train or bus stations.
Tourists and families will likely find Gatwick better than Heathrow with more flight options, particularly on charter flights, and at cheaper prices but business travellers will likely find Heathrow the better airport due to convenience and range of scheduled flights from the top airlines of the world.