Things are certainly more convenient in Wacom World now that they connect via USB and you can use the same tablet on either platform.
The model you get doesn't make much difference; they all have the same basic functionality for retouching.
This is one case where size does matter, though, so I'd say get the biggest one you can conveniently afford. I use a 12x12 at work and a 9x6 at home, and I'm noticeably more productive on the 12x12 -- able to spend more time retouching and less time moving the screen area around. (Retouching is one of those activities in which I find I do better if I can get "in the zone" and stay in it.)
The ideal, IMO, is to have a tablet with a physical size similar to your monitor's work area, so your hand movements map more naturally to what your eye sees the cursor doing. I guess the ultimate along those lines would be the tablets they make with an LCD monitor built into them, but those are mind-bendingly expensive...