I commute by coach to London nearly every day. If you want a (slightly) faster service, the Espress is the way to go. There aren't as many seats (no double deckers), but they don't stop at Junction 6, drive a little bit faster, and only stop when someone has buzzed (good for those traveling from the town centre).
UPDATE: I now have a different job and don't use the buses as often. The competitor to the Espress, the Oxford Tube, bought all new buses a couple of years ago, so the competition is intense. Between the two companies, this route is apparently one of the busiest bus routes in Europe. read more