Bike box or bag?

I'm flying off to Ireland for xmas and wanted to know if i'd be better off with a bag (this is the one i had in mind http://www.wiggle.co.uk/p/Cycle/7/dhb_Marsden_Wheeled_Bike_Bag_And_Wheel_Bag_Set/5360034155/) or pack it with foam/bubble wrap and put it in a cardboard box, i'll be sending it by Royal Mail as the allowance on the flight is something ridiculous like 32kg including all my other baggage, whereas RM is £14 for 20kg.

Any thoughts appreciated (helpful thoughts especially) =P

ETA - If I went for the bag, would I have to put it in a box still, or just stick a label on it?

did you go with the bag or box? whats your opinion on it? Im heading off o/s with my bike and Im in 2 minds which option to take