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Ive used the 12 and find it cumbersome due to small size complexity of the controls. The 24 is the opposite, HUGE, HEAVY, NON-Portable. And frankly, made for someone that does this at their work station as a living. It's like an editing station on steroids.
And if your just coming from a "mouse" setup, the $2,500.00 is a bit of overkill in my eyes. The Intuos4 Medium is a great tablet. Price pointed out in the upper 200 range and certainly gives you all the options you'll need by way of editing.
The ONLY disadvantage to the 12/24 vs. the Intuos line is the screen calibration=workflow etc..
If you have a place where you can view both of these units in operation, that would certainly clinch it for you, but from mouse to a 24? Nope.