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The battery is a lithium-polymer. The faster you charge one, the shorter its lifespan.
Lithium-polymer batteries have built in safeties to prevent over charging and charging too fast. At a certain point (no idea what that might be with the iPad) it doesn't matter how many more amps a charger can supply; the iPad will charge no faster.
This is why it is safe to charge your iPhone on the iPad's charger, even though that charger supplies twice the current.
It is possible that the iPad will lose a bit more battery life by charging at the maximum rate allowed by the circuitry, but it's unlikely to be significant over the expected life cycle of a mobile device.
Laptops now, with their four to five year life cycle, that might be worth considering; providing you really do use one mobile and not just as a desktop replacement.