Regarding pens' compatibility I have a very limited experience with that. So I would simply advise you not to assume that pens are interchangeable.

My current config is an iMac 27 and Intuos 4 Large. The width to height ratio of both are almost the same which means that a square drawn on the tablet translates into an almost perfect square on the screen. Overall this Intuos 4 is good but it also has a downside -- the buttons on the side of the tablet are very stiff. I had a chance to try a new Bamboo and its buttons are much softer and easier to press.

Also I wouldn't rule out Bamboo altogether. I would look for a chance to test drive it if at all possible.

Ah, and the tablet itself seems to last forever. It is the pen which is easy to break. First the pen's tip lasts for several months give or take. They are replaceable and there are several tips bundled. Also the pen's side button is prone to break. And the worst thing which can happen with the pen is to let it fall on the floor and have it land right on its working tip. Too much pressure for the pressure sensor I suppose :) that was when I learned the hard way that wacom pens are not universal.