Catman--cost comparison depends upon how many in your party. Cost was 10 pounds per person into Paddington, and then Cab to final destination. We were at MayFair inter-Continental, which cost us about another 8 pounds (4 people with luggage). So, done that way, it cost about 50 pounds for 4 of us.
When all 4 of us took cab from MayFair to Heathrow to return, it cost us 45 pounds with tip, but we were very cramped into a cab for 45 minutes or so. The train was great,clean and lots of room. Would probably do the train next time both ways, with enough time.