How long does it take to get from Heathrow Airport to Paddington Station?

My Virgin Atlantic plane is landing at 9:25am and I want to catch a train out of Paddington that day. Is it safe to buy a ticket for a 10:45am train, or should I go with a 12:45pm train instead? I need to go through customs and pick up baggage, and I am an American citizen. I'm worried that the first train is too early and the second one seems too late!