I was thinking of doing the same thing myself, however I went to the store and played with 13HD and the 22 HD. In my opinion it would be worthwhile holding off and buying the 22 HD when you have the cash. The 13 HD is cool as, but I think it is too small.
I played with them for a while in the store and I found that the 22HD is the one that I would like to move on to.
If I was you I would buy the Intuos 5 big size with the touch capability. Here it is about 300 USD.
The reason I say this is I have a iMac and I am used to working on a big screen, even though it is using the tablet and looking at the screen.
I think I would find it very difficult to work on the 13HD after having used a big screen for so long, thats's why I would like the 22HD.
Hope this helps.