Also, the 10 W vs. 12 W charger should make little difference, i.e. you should still be getting no more than a few hours charge time on your iPads regardless of the charger wattage being used.
If that were true, then there would be no reason for Apple to supply the larger chargers in the first place. It all depends on whether the device is capable of using that extra wattage, or more accurately, drawing more current.
If we were talking about an iPad Mini, which comes with a 5W charger, then there would be no difference between the charging rate using a 10W or 12W unit. The Mini can draw only a little more power than the 5W it's supplied with. It would probably charge slightly faster on the larger units, but since it's limit is less than what either can supply, there would be no difference between them.
However, the iPad Air can draw more power. This will have the largest effect when the charge is low, and the iPad is pulling all the current it can (up to it's limit). As the battery charge goes up, the current demand decreases.
So, for the first part of the charge the 12W power supply will be faster. Say (
made up figures) from 10% to 40% it might be significantly faster, from 40% to 60% only a little faster, and no difference from 60% to 100%.
This would not be a linear. It would be a steep initial curve, tapering to a gradually to a plato.
The difference will be even larger if the iPad is on, and larger still if in use. .4A is only about 25% more current than 2A, but if some of that, say half, is going to keeping the iPad running then you have 1A vs 1.4A, which is 50% more current available for charging.
Would this result in the dramatic difference Mixz1 is seeing? I don't know. I don't' have real figures to play with. But it would certainly make a bigger difference than the 10W vs 12W (2A vs 2.4A) seems to imply.
As a side note. I thought the 4th generation iPad came with a 12W charger. It was the first to move up from the 10W charger, a response the many complaints about how the iPad 3 (with it's larger battery) took so much longer to charge than the iPad 1 and 2.