I would think that the easyBus would be the easiest and cheapest option. The fare starts at £2.00 if booked online, no need to hold onto your luggage on the train (there's no real luggage space on the train), and it takes you right to the terminal. Their schedule is good too, and I think it takes a bit longer than the train but then you don't have to catch a train from the train station.

You don't need to be flying on easyJet to use easyBus, by the way.

Have a look here: http://www.easybus.co.uk/