Well, for one, the A5’s active area is a little smaller than 6 x 9. I use a 9 x 12 Intuos3 and it provides a much better drawing area. It’s twice as sensitive, which you’ll only notice a little, but way more accurate. The Intuos has a much better digitizer under the pad with twice the lpi, or tablet resolution. That combined with the larger drawing area makes a big difference in the feel of it, much more fluid strokes like real drawing. The tilt function isn’t really used in ZBrush for my workflow. That and extreme sensitivity are more important with digital painting.
If you just get a 6 x 8, you’ll notice twice the accuracy. If you get a 9 x 12, you’ll have about 3 to 4 times the accuracy. If you get a 12 x 19, it’ll be almost as accurate as drawing on your 24" screen with a lead pencil! So the difference is more about precision, not so much the tilting function (with ZB). The sensitivity is better, but 512 levels is still a lot. It’s also more durable and built for pro studios.
Or just get a Cintiq 20WSX and make me REALLY mad at you…