That's one of my two main reasons that sets the two airlines apart. The other reason?
It is a very stupid policy by AirAsia. It has been mentioned before by the media - for example, here and here - that the cost is only about 25 cents per passenger to use the aerobridge based on a full load of passengers. Apparently, AirAsia looks too stingy to fork out this amount, true to their low-cost business model, but in the process they inconvenience and alienate a significant segment of those who want to fly with them.
If I have a choice, I wouldn't use AirAsia and I know that I do have a choice on the Penang-Singapore sector.