Hello,
We are going to be staying in the South Kensingston area and wonder whether it would be faster to take a cab (or car service) to the airport on a weekday morning or take a cab to the Paddington station and then the Heathrow Express. We have an 11:15 am departure from Heathrow, Terminal 5.
Thanks!

South Kensington tube station is on the direct Piccadilly line to Heathrow. I suspect it would take about the same or less time as a taxi & Heathrow Express or the less expensive Heathrow Connect. Going that direction I suspect it'd be less crowded than coming in and certainly less expensive than cab/train. I think I'd opt for the tube, get on once, get off at T5.

One more vote for the Tube.

if you are truly in South Kensington and near Gloucester Rd or South Kensington tube stations - that would probably be as fast or faster than by car. Flat fastest would probably be a cab to Paddington and the train to LHR - but not enough faster to make up for costing 6 or 7 X more.

The Tube is definitely the best option - fast and cheap and it will take you straight to T5. But, if you are 3 or 4 travelling, a taxi would also be a good option (a little slower though).

Benterry: Sorry, but a taxi would NEVER. Be a good option. If one is going by road, a pre booked car service would take exactly the same time for half the cost.

When I was saying about a taxi, I meant about a pre-booked one, not a black cab. Of course a normal taxi that you simply pick-up from the street will be more expensive.

You can hop on the tube/Piccadilly line from Knightsbridge, South Kensington, Gloucester Road or Earl's Court and be at Heathrow in about 40 minutes without changing. Just look at the front of the tube when it comes into the station as it will tell you which LHR/Heathrow stations it goes to. If you buy an Oyster Card which is reloadable and returnable,you can get a one way for less than 8 pounds a piece.
The Paddington Express is a completely different thing as it is a more expensive and faster (about 20-25 minutes)train than the usual London Underground.
If there are 3,4 or more of you,I would recommend looking into online the limo service called: JUSTAIRPORTS.com
They will pick you up either end for a very decent and inexpensive compared to a cab.

Thanks everyone!

Take the tube. Last summer we were at a hotel near Earl's Court. Easiest experience getting back and forth to an airport for all my trips abroad -Europe and the U.S.

Take the tube. Taken it from Earls Court to T5 on two separate trips on a weekday morning and had no problems or time issues. Straightforward and easy.