As a local to LHR
, and previously LGW
, I'd say the main factors in your decision should really be the time at which you will be transferring, and how long you've got to make your connection.
I've got a National Express coach between LGW
North Terminal and LHR
Terminal 5 before that's taken only 40 minutes; during the morning or evening rush hour this time could more than double, during which times I'd take a train. Although the train is a bit more involved with multiple changes, it'll give you a much more reliable connection time during peak hours than sitting in a traffic jam.
Conversely, on weekends, our wonderful railway and underground systems are often subject to engineering works, delays and cancellations so the coach would be your best bet.
I wouldn't go near a taxi personally as it'll get stuck in the same traffic as the coach. Some of the coaches even have free WiFi if you're that desperate to get on the Internet.
"Our 319's are very reliable. They get fixed very quickly."