I had the same decision when planning to purchase the high end 13" or the low end 15". In the end I decided that the ATI graphics card and quad core was worth the extra cash.
In the end it will make autocad 2D and 3D work much faster, and help future proof your laptop. Plus the extra screen resolution and size will make using your tool palette a whole lot easier. Unless you will be using an external monitor most of the time.
In summary, the high end 13" will perform fine, but the 15" is a serious improvement. Pick whichever laptop will suit your needs and budget.