Now that all is said and done, if I had to do it over, I'd take the bus. Granted it would be slower but it would be (at least for me) much easier. The train involved lugging out to a shuttle bus, getting to the station, standing on a liine to turn an online voucher into a ticket, waiting for a train, then taking the tube -- all the while dragging my luggage around.
Had I taken the bus, it would have picked me up in front of the terminal and taken me to Victoria. Much easier...