I agree, if there are three or more people and you can fit into one cab, that's the way to go - except at rush hour when the train still goes full speed! For one person, the train is much less expensive, somewhat less convenient, and probably a bit faster. It also depends somewhat on where you're going in London, relative to Paddington, the tube, and the train-coordinated buses.

A warning on cabs, by the way - if you're going to someplace near Heathrow, vs. into London, a London metered cab can be VERY expensive because it's off the meter. DO NOT set foot in one until you have agreed on a price to your destination. I paid £40 to Egham last month - an expensive lesson, but I had no other way to get there quickly.

Less expensive, if you have the time, is a shuttle to any nearby hotel and a local taxi from there. I have not done this myself, but it was recommended by someone who should know. Can anyone comment?